Declan is the youngest Duelist in history to be certified to Duel in the professional circuit, having done so at age 15. Despite his early introduction into the series, Declan's name was not mentioned on-screen until episode 8.
Declan is a thin young man with pale skin and grey hair, the front layer being a lighter grey than the back. His hair is arranged in a simple style, flicking upwards at the ends. Declan's eyes are purple, and he wears red-framed spectacles. His ears are both pierced with black stud earrings. Despite his status as the CEO of the Leo Corporation, Declan dresses rather casually, wearing a dark-blue sweater, white pants that end just above his ankles, and white shoes (apparently without any socks beneath them), along with a long red scarf wrapped around his shoulders with the ends trailing to the small of his back. When he was monitoring the You Show Duel School in person, he concealed his identity beneath a large-hooded sweatshirt.
In general, Declan has a grim and serious demeanor, but he takes an interest in Yuya Sakaki. He is utterly polite without fail, save rare moments of a loss in composure. Despite his advanced skills, he doesn't seem to be overly arrogant, and is actually quite respectful to Yuya's father Yusho Sakaki, fiercely defending him when he was insulted in a rare moment of emotion. While he does not consider empathy as beneficial on the battlefield, he does seem to enjoy himself while Dueling, and even requested Yuya to demonstrate Yusho's Entertainment Dueling.
Despite his declared rivalry with Yuya Sakaki, he does not show any open dislike for him, and in fact defended him from false accusations of attacking Leo Corporation employees; this demonstrates that he may hold a degree of respect for Yuya. Declan has also mentioned that he does not care very much for his father, Leo Akaba, and he demonstrates an intense dislike of his father's ambitions, given the suffering that they have caused. Declan is also pragmatic without regard for the actions of potential allies, offering an alliance with Shay and Celina despite their attacks on his students.
In his youth, Declan was more reckless, crossing dimensions (though unaware that he had done so at the time) to find his father after he had disappeared and exposing his hiding place to save Celina. He also spoke far more brashly (in Japanese, he referred to himself as "boku" in contrast with his current self who refers to himself as "watashi", which is the polite form), demanding answers from his father.
Three years ago, Declan appeared inside Duel Academy via a strange teleportation device. He wandered the halls until he noticed Celina being chased by faculty members. When they cornered her, he watched from atop a wall behind her as she Dueled against them, and was surprised to see that their holograms had mass without an Action Field. He jumped in, Summoning "D/D Cerberus" to defend her. He then offered her his hand, which she took, and pulled her up over the wall. They fled to the harbor, where they were caught by Leo Akaba and several Obelisk Force members, to Declan's surprise. Celina mentioned that Leo wanted to destroy the Xyz and Synchro Dimensions, mentioning that the attack on the Xyz Dimension was about to start, prompting a confused Declan to protest that Leo had not mentioned anything to him or his mother. The Obelisk Force squad then took Celina away, and then one of them placed a miniature teleportation device on Declan's wrist, which sent him back to the Standard Dimension and then self-destructed.
Pre-Arc League Championship
After Yuya Sakaki defeated the Sledgehammer using the Pendulum Summon mechanic, Declan was briefed on the Summon and on Yuya himself by Claude. Intrigued by the Pendulum Summon, Declan asked Sylvio to learn what he could about Pendulum Summoning, no matter how small the detail and to acquire them, if possible. Sylvio successfully acquired the cards, but wanted to use them to Duel Yuya. As Claude protested from where he was watching the Duel, Declan arrived and assured him that it was all right. He advised Sylvio on the use of Yuya's Pendulum Monsters from behind the scenes throughout the Duel. After Sylvio's loss, Claude stated that they would need to use other means to seize the cards immediately, but Declan again reassured him that it was all right for now. He looked at a video feed of Yuya, thanking him for putting on a show and murmured Yuya's name.
A week after the first Pendulum Summon, his mother Henrietta arrived back from a business trip. Declan wasted little time in informing her about Pendulum Summoning and Yuya Sakaki, and stated that they'd almost achieved a complete understanding of the Summon. He also informed her of Sylvio's supposed attack at Yuya's hands. Declan admitted to being worried that, despite the feat he had performed, Yuya only had a 50% win-loss-ratio, and he was the son of Yusho Sakaki, who had disappeared three years previously and possibly allied with enemies of Leo Duel School. Later, he observed the Duels between the You Show Duel School and LDS, remaining hidden and concealing his identity beneath a hooded sweatshirt. After Yuya had defeated Dipper, Zuzu Boyle had been defeated by Julia and Gong had tied with Yaiba Todo, Henrietta angrily proposed a tiebreaker between the students that had won; Yuya and Julia. Before the situation could progress any further, Declan revealed himself and stated he would put an end to it.
None of the students present recognized Declan, his mother commenting that they didn't know how strong he was. Declan requested that she and their students stay quiet and watch, and that they not cheer any further. Declan allowed Yuya to choose whichever Action Field he wanted, so Skip Boyle (the only You Show member aware of Declan's identity) picked Yuya's favorite Field, "Acrobatic Circus." In exchange, Yuya let Declan go first. Declan proceeded to use three "Covenant" Continuous Spell Cards to Fusion Summon "D/D/D Flame King Genghis" on his turn (this is his first use of a Fusion Monster to be known, and there were no records of him using them during any of his past Duels), then set the rest of his hand, but leaving him apparently guaranteed to take 3000 damage on his next turn. When Yuya attempted to retaliate, Declan activated another "Dark Contract" card that would cause him to take another 1000 damage during the next turn. Yuya was able to counter that attack as well, but Declan was well-prepared, using an Action Card to protect "Genghis." Yuya was given the option to end his turn and theoretically win, but hesitated. Declan observed that Yuya's kindness was causing him to hesitate, though he called it useless in battle. Yuya Set a card, then Declan negated and destroyed his "Dark Contract" cards with "Contract Laundering," replenishing his hand in addition. Yuya admitted that he wouldn't have been happy if he'd won from that, as he wouldn't have gotten a chance to show off his father's Dueltaining. Declan asked if he was referring to Yusho Sakaki, and Yuya was surprised that Declan seemed to know his father. Yaiba mocked Yusho from the sidelines, prompting Declan to roar at him to be silent. He apologized for his outburst, explaining that he respected Yusho from the bottom of his heart. Declan then proceeded to Synchro Summon "D/D/D Gust King Alexander" and Xyz Summon "D/D/D Wave King Caesar" on his next turn, much to the shock of the onlookers. He destroyed all of Yuya's monsters, despite Yuya's use of "Hippo Carnival." Yuya proceeded to Pendulum Summon on his next turn, using his "Odd-Eyes Pendulum Dragon" in combination with "Performapal Fire Mufflerlion" and an Action Card to destroy all three of Declan's monsters. Declan used the effect of "Caesar" to revive them, though at the cost of taking damage again on his next turn, and then he Tributed them with "D/D/D Resource Management" to add his "D/D Savant Galilei" and "D/D Savant Kepler" to his hand - Pendulum Monster prototypes that he proceeded to activate on his next turn. He congratulated Yuya after Yuya claimed that he looked forward to seeing whatever surprises Declan had in store, Declan asking Yuya to confirm whether Pendulum Summoning was unique with his own eyes. Declan then Pendulum Summoned three copies of "D/D/D Doom King Armageddon", destroying all of Yuya's monsters again and reducing him to 350 Life Points, also discovering, to his surprise, that destroyed Pendulum Monsters were instead sent to the Extra Deck instead of the Graveyard. Yuya demanded to know where Declan had acquired Pendulum Summoning, but Declan refused to answer. Yuya Pendulum Summoned his destroyed monsters back, along with a new one, "Performapal Partnaga," and attempted to destroy all three "Armageddons", but the destruction of one simply activated the effects of the others, which gained the ATK of the destroyed "Armageddon." Before Declan could counterattack, his Pendulum Monsters' effects appeared and reduced their Scale, destroying his remaining "Armageddons." He burst out laughing in surprise, realizing that Pendulum Summoning was not complete and that he had seen the possibility for further evolution in Pendulum Summoning. His mother then called out his name after hearing about the disappearance of Marco and Claude briefed Declan on what had happened. Declan gave Yuya the win, and when asked, told Yuya his name before departing. Arriving back at LDS, he learned that Marco had been involved in a Duel with an Xyz user. As they'd been with Yuya at the time, and Yuya had not demonstrated the capability, this ruled him out as the culprit. Declan ordered his men to search for Marco after being shown Marco's cards and Duel Disk.
Following this, Declan anonymously requested that Yuya be allowed into the Junior Youth Championship. Yuya, however, declined, and won his first Duel of the required four to gain entry normally. Claude later informed Declan of this, and Declan smiled, commenting that it seemed he'd done something unnecessary, that he should have expected it of the originator of Pendulum Summoning, and he asked if Yuya had Pendulum Summoned and if there was any change. Claude replied that there wasn't and admitted that he was bothered by elevated levels of Fusion Summoning that were occurring. The readings were stronger than normal, and Declan compared it to the abnormally strong Xyz Summonings that had occurred recently. He ordered it to be monitored. His mother later approached him with two cards that had been sent anonymously to LDS that morning depicting Marco and Herk, one of LDS's aces. Declan mused that the previous day, another strong Xyz Summon had been detected, and deduced that the two had been sealed inside the cards, though this was the first time he'd witnessed such a phenomenon. Though Marco and Herk were alive, they couldn't be saved with LDS's current power. He commented that this did raise one question - there had been there Xyz-related attacks, yet the first victim - Sylvio - hadn't been sealed in a card, suggesting two perpetrators. Before they could continue their talk, Mr. Sawatari barged into the office furiously, asking why they were loafing around. Despite Mr. Sawatari not having an appointment, Declan acknowledged that the rumors of the third attack were true. Mr. Sawatari angrily asked why they weren't taking action to catch the perpetrator, pointing out that his son had testified that Yuya Sakaki had attacked him. Declan asked if Sylvio was still saying that, and Mr. Sawatari admitted that he hadn't seen Sylvio since he'd left the hospital, but hadn't heard Sylvio say differently. He asked why they were letting Yuya run free, despite Henrietta wanting to handle it herself, and wanted to inform the police. Rising from his chair, Declan warned him that it was under investigation. Mr. Sawatari insisted that Yuya was the perpetrator, but Declan forcefully told Mr. Sawatari that Yuya was no criminal. He vowed to catch the real criminal, until then, he'd like Mr. Sawatari to watch quietly. After Mr. Sawatari had left the office in a huff, Declan ordered Claude to raise the level of alert in the inner city and analyze the cards of Marco and Herk to save all of the city's Duelists. He later learned that the strong Fusion Summoning readings were continuing, and mused that the person using them must be winning as well.
Later, Declan practiced Pendulum Summoning again, this time with the effects of "Galilei" and "Kepler" updated, and he successfully Summoned all three "Doom King Armageddons." Imagining himself facing Yuya and "Odd-Eyes Pendulum Dragon", Declan reached for his next card, but before he could continue his move, he was advised to stop, as they had cleared the day's benchmarks and the experiment was a success. Claude congratulated him, stating that now they were one step closer to creating Leo Corporation's own Pendulum Cards. Despite this, Declan considered his own inferior in terms of activation energy output to Yuya's Pendulum Monsters, believing them to be incomplete and unusable in the tournament. He gave Claude until the tournament to fix the problem, as they could not underestimate their competition, no matter where it came from.
When Claude was complaining about using students to draw out the Duelist that had been attacking their students, Declan arrived and ordered them to continue monitoring them, confident that whoever it was would show themselves. He witnessed Shay Obsidian admit to attacking the LDS students and the start of his Battle Royale against Julia, Kit, and Dipper. When Shay defeated all three of the students and he ordered the other LDS students that had arrived to bring them their boss, Declan replied that he was right there if Shay wanted him, and his employees parted as Declan walked towards the Xyz-user. Declan explained to Shay that he was the current Leo Corporation President, and Shay immediately challenged him to a Duel. Declan requested to know why Shay was attacking them. Though Shay refused to answer, Declan had deduced from his actions and Julia's prior testification that Shay was trying to rescue his comrades by using Declan as a hostage. Shay confirmed that his attacks had been to lure Declan out to persuade his father to release his comrades. Declan claimed that he doubted his father valued him that much, but agreed to Duel Shay under conditions: Shay must enter the Junior Youth Championship and fight certain Duelists for him, informing him in a roundabout manner of Sora Perse, a Fusion user, and Yuya, the user of the Pendulum Summoning that Shay would likely be unfamiliar with. Declan explained that he too wanted to fight against his father and clarified that if Shay won, he could do whatever he wanted. He departed with his men afterwards. Claude later confirmed that Yuya had successfully entered into the competition and that Shay had been wiped from Julia, Dipper and Kit's memories, and entered into the Junior Youth tournament. Declan mused that all that was left was to see how much power Shay would display, and he looked through his Lancers Selection document, muttering that the same question applied to the rest of the Duelists in the folder - which of them would become the lances to fight for their world?
At the beginning of the tournament, Declan made one of his first public appearances in a while, much to the delight of his fans. He, his mother, and Riley watched the opening proceedings, observing the entry of both Yuya and Shay, and after Yuya's brief mess-up and subsequent speech regarding Yusho Sakaki, Declan led the applause. He watched Riley's first Duel against Allie, and congratulated his little brother after he won.
Before Sylvio's Duel with Yuya, Declan gave him his new "Yosenju" cards, including some Pendulum Monsters, and taught him an anti-Pendulum combination that would return Pendulum Cards to the Deck. He monitored the Summoning, which was successful, as was Sylvio's use of the technique. He was pleasantly surprised when Yuya Fusion Summoned using a Pendulum Monster as the Material, pleased that Yuya had already evolved so far. After Sylvio's loss, he ordered Claude to retrieve his Pendulum Cards and analyze them for mass-production.
Before Shay's Duel with Sora Perse, Declan arranged for the Action Field "Future Metropolis Heartland" to be used to light Shay's fighting spirit. Declan also had a Pendulum Card made for Shay, who rejected it. While this annoyed Claude, Declan was fine with the rejection of his gift, saying he was more interested in Shay's abilities rather than the potential of Pendulum Summoning. He was quite surprised when Shay performed a Rank-Up-Xyz Evolution during the Duel. When Shay called out "Raidraptor - Revolution Falcon," he told Riley to watch their future at stake. After Shay defeated Sora, he told him not to take further action in the eyes of the public, as Sora would be useful for experiments to understand Duel Academy.
Sora escaped during the night, and Declan coordinated the search for him, finding him in Central Park with a man who resembled Yuya Sakaki. Shay explained that this was his comrade, Yuto, the second Xyz user that Declan had suspected of attacking LDS. He refused to let Shay assist Yuto, and they simply watched the Duel between Sora and Yuto, and the subsequent arrival and joining of Yuya. After Sora revealed himself as an agent of Duel Academy, his Duel Disk transported him away. Declan watched as Yuto explained to Yuya about the four Dimensions, growing angrier as he heard about the attacks on Yuto's homeland, and explaining that it was due to his father, Leo Akaba's ambition that the tragedy had occurred. Before he could learn more, the security camera they were accessing was damaged. Declan still refused to let Shay go, warning him that he would protect his city his way. When they detected a Synchro Summoning reaction, Shay explained that Yuto must be Dueling the Pawn of Fusion. Claude later found Yuto's Duel Disk, and Declan ordered it brought to him as he looked out over Paradise City. Claude did as ordered, and Declan proceeded to scan it for information.
Declan witnessed Yuya's next Duel with Iggy Arlo, suspecting that Yuya had possession of Yuto's "Dark Rebellion Xyz Dragon"; his suspicions were confirmed when Yuya Xyz Summoned it. When a stunned Shay turned to go and find Yuya, Declan reminded him of the tournament's purpose before he left. Declan then heard Nico Smiley announce that Dipper had lost his Duel to Reed Pepper by default due to absence. This confused Declan, until he was informed that a Fusion Summoning reaction had been detected. He was shown security camera footage that caught Dipper being sealed inside a card by Celina, whose face he recognized.
He then learned that Celina had watched Kev Ravenwood's Duel and realized that Kev would be Celina's next target, so he devised a plot to use Kev as bait to lure Celina, which worked perfectly. Declan immediately told Kev to leave, and as the latter turned to do so, Celina chased him, until Declan stopped her by grabbing her arm. This prompted Celina to threaten to take Declan out first, until Barrett, Celina's cohort, challenged Declan instead. Declan prepared his Duel Disk, ordering Claude through a communication link to seal off the area. Declan inferred from Barrett's use of Medal Counters and willingness to take risks that Barrett had extensive combat experience, which Barrett confirmed. Though Declan eventually took damage from his own "Covenant with the Swamp King" card, earning a respectful comment from Barrett, this damage ultimately sealed Declan's victory, as he proceeded to dispatch Barrett using "D/D/D Tell the Sniper Overlord" and "D/D/D D'Arc the Oracle Overlord". Celina turned to leave, until Declan told her to wait.
He commented that she wasn't Zuzu Boyle, and Celina asked who Zuzu was. Declan explained that Zuzu was someone from Standard that resembled Celina and introduced himself, asking her if she remembered him and if she had been sent by Leo Akaba as a vanguard for an invasion. Celina denied working with Leo, having come to Standard of her own free will, and she admitted that she remembered Declan too, as she'd been trapped in that castle since Declan's appearance in her world. Declan explained that he was opposing Leo and offered an alliance, but Celina refused, explaining that she was after the Xyz users in Standard under the belief that defeating them would make Leo Akaba acknowledge her abilities. She gave Declan the card containing Dipper's soul, and Declan informed her that neither Dipper nor Kev were her target, though he claimed to not know the Duelist from the Xyz Dimension who had defeated Sora. Declan warned Celina that he would be considering her an enemy even if she wasn't working with Leo and would take appropriate action. Barrett, overhearing this, activated the forced return function of his Duel Disk, returning to the Fusion Dimension, but alerting them to Celina's location. Celina explained that there would be people from Duel Academy coming after her, which would cause more than just a disturbance, but claimed before departing that once she'd defeated them and the Duelist from the Xyz Dimension, Leo would have no choice but to acknowledge her abilities.
To prepare for the impending invasion, Declan organized the third round of the Championship as a Battle Royal between the 16 remaining competitors, as he was worried that simply warning the citizens would cause a panic. He intended for the Top 8 Duelists from the Youth Championship to intercept the invaders, and for the Junior Youth Duelists to take them out. He instructed Shay to monitor the Junior Youth Duelists and seek out those who could join the Lancers in case of an emergency, to which Shay argued that he'd have to abandon those orders if the Youth Duelists all got defeated. Declan later returned to his control room to monitor the proceedings. He cut the video feed from the Ancient Ruins Area of the Action Field to the stadium when Obelisk Force appeared, followed by the Volcano Area when Celina showed up, then ordered the Youth Duelists to move out. When the Obelisk Force appeared before Celina, he ordered the Youth Team to be sent to the Volcano Area. The Obelisk Force whittled down the Youth Team until only Kev remained, though he was saved by a Duelist from the Synchro Dimension who wiped out the entire Obelisk Force. Declan realized from this that the Synchro Dimension wasn't their enemy and that LDS weren't skilled enough to resist Duel Academy. He theorized that the Duelists from Yuya's generation might be able to become stronger due to entering the new stage of Dueling that Pendulum Summoning had brought about.
Reiji faced Yuya in an Action Duel in the special manga chapter. He used relentless tactics to quickly Summon high-Level monsters and was skilled with the use of Action Cards. In this chapter, Reiji is portrayed as a rather serious Duelist, and he claims that he never loses Duels.