Syrus Truesdale (Sho Marufuji in the Japanese version) is the Jaden Yuki's best friend, he follow him in every adventures. He is a former student of Osiris Red, then Ra Yellow, and now Obelisk Blue (In the season 3 and 4). A decent duelist who doubts himself because his older brother Ryo Marufuji, but the good influence of Jaden and his others friends make him more confident.
He is voiced by Masami Suzuki in the Japanese version of the anime, and by Wayne Grayson in the English version of the anime.
Syrus is a shy and unconfident boy. However, he not only grows into becoming a better duelist, but a more brave and confident boy.
He is a very loyal boy, often thinking of Jaden as a big brother figure.
Syrus is considered to be the "weaker" younger brother of Ryo Marufuji who attends Duel Academia, originally residing in the Osiris Red dormitory along with Hayato Maeda and Jaden Yuki. Syrus has made progress as a duelist throughout the series. From the onset, he faced Jaden in preparation for a tag team duel, but, priding himself on finding a way to outdo his roommate, overlooked the abilities of his own cards. It was for this same reason that Ryo forbade him from using the "Power Bond" card, for Syrus was unable to cope with its potential drawback.
During the second season, Syrus is given an opportunity to advance to Ra Yellow, should he be able to defeat Ran Kouchou. Feeling that he should not shoulder his brother's legacy, Syrus gives his "Power Bond" card to Chazz Princeton to pass to Jaden before the start of the Duel, though Jaden convinces him to put it back in his deck after the duel.
Syrus develops a rivalry with Tyranno Kenzan over which of the two is better suited to being Jaden's best friend (in the Japanese version, it is over who has the right to call him "big brother"). They constantly argue and fight, prompting their repetitive separation by Alexis Rhodes. By episode 64, their rivalry comes to a temporary halt, when the two duel to settle matters once and for all. Before the last blow is dealt, Kenzan discovers that Syrus went without food or sleep to search for Jaden, who had gone missing since his defeat at the hands of Edo Phoenix. Seeing Syrus' devotion to their friend, he chose not to activate his "Jurassic Impact" card, which would have wiped both players out simultaneously.
During the field trip to Domino City, Syrus teams up with Kenzan in a duel against the first duo of the Four Monarchs, Koorimaru and Ikazuchimaru. Although the two are pinned down by their opponents, who tuned their decks specifically to combat their cards, they end up pooling their efforts to combine their monsters into "Super Vehicroid Rex Union". Despite their teamwork, however, Koorimaru ends the duel by wiping out all players but himself, and promptly drags them to his master's lair, from where they are transported to Kaiba Land as bait for Jaden and Edo by order of Mizuchi Saiou.
As he observes his brother's actions following his "rebirth," Syrus notes Ryo's newfound disrespect for his own cards and opponents. He dismisses the barbaric persona, refusing to believe that his brother has sunk so low. Determined to bring his brother back to his senses, Syrus acknowledges the inevitability that the two of them will face one another. During the Genex Tournament, Syrus and Ryo duel, they are hooked up to electrodes, just as Ryo and Mad Dog Inukai were in the Underground Duel. Despite his best efforts and Jaden's secret move to place "Power Bond" back into his Deck, Syrus loses, but he does not give up in hoping that his brother can be returned to his former self.
Shortly after the beginning of the third year at Duel Academia, Syrus advances to Obelisk Blue. As a result of survival duels being called for by Professor Cobra, Syrus becomes self-conscious of his worthiness to wear the outfit of the elites, and sets out to prove himself. In the Japanese version, he also wishes to adopt his brother's discarded title, and thus merit the name of "Kaiser Sho" (カイザー翔, Kaizā Shō). Inspired by Jaden's match with Austin O'Brien, however, he decides to remain in Ra Yellow, and sets aside his goal for the future. (In the English version he quits the Obelisk Blue dorm stating that blue was not his color and he left Obelisk to protect Jaden). After Duel Academia is transported to the first alternate dimension, Syrus falls to the Duel Zombies and becomes one himself.
Later, when Jaden and company venture into another alternate dimension to rescue Johan Andersen, Syrus is the only one of Jaden's closest friends who isn't sacrificed by Brron. After Jaden wins, Syrus verbally scorns him saying he wants nothing to do with him and up leaving him. While separated from the others, Syrus decides to act as an observer, but later reunites with Edo and Ryo when Jaden is released from his possession. After Ryo loses to a Yubel-possessed Johan, Syrus watches Ryo die and fade into particles of light before breaking down in tears.
In season 4, Syrus advanced to Obelisk Blue again. Syrus later attempts to finish Ryo's duel with Makoto Inotsume after Ryo becomes too weak to duel, but immediately regrets his decision later as he begins to develop the same heart problems as Ryo because of his use of the "Cyberdark" Deck. However, Syrus manages to overcome his problems and defeat Inotsume, thus inheriting Ryo's Cyber Dragon Deck later on and promises to improve it. However, he still later falls to him, and is absorbed into the World of Darkness. It is shown within the World of Darkness that Syrus fears that he will be unsuccessful at creating a Cyber Legacy Pro League with Ryo (as it is his brother's dream) and that he will have to be the successor of the Cyber Legacy when Ryo becomes greatly ill (as he has his limits). He also fears that he will be unable to make this dream come true due to few to no people and sponsors being interested in it. It is also shown that if Ryo dies when Syrus is still unsuccessful at creating a great Pro League, then he will feel relieved that he is finally released from trying to create something he could not succeed at creating. He is revived after Jaden defeats Darkness.