Shinji is a good hearted, friendly, but naive idealist who sees the Rider War as just meaningless chaos. He is willing to go any lengths to prevent the Riders from killing each other. Meeting Riders like Ren Akiyama however, who fight for a selfless wish has Shinji decide to work with them. His interaction with other Riders, particularly Ren and Kitaoka, leads them to question their resolve and their willingness to fight and kill in the war. He has even seemingly managed to tame and befriend Dragreder itself.
Right before his death though, he finally sees that there is no justice in the War, and thus sees that he has to fight and kill to stop the fighting. He never got to go through this traumatic phase of his life.
Despite being unwilling to fight and kill Riders, he gives his all when he does fight at all, especially with Mirror Monsters. This is shown in him taking a deep breath before entering the Mirror World, and he often pumps his fist and shouts "Alright!" in battle. Despite not being very smart, he's capable of thinking of out-of-the box strategies for taking down Mirror Monsters, leading Tezuka to label him as "unpredictable."
Shinji was aiding in the New Group's investigation behind the "missing persons" crisis that had with the latest victim being Ida Megumi. While checking on the list of missing persons, Shinji finds a clue and investigates the apartment of Kōichi Sakakibara, a man who mysteriously disappeared two months ago. Upon coming across a deck of cards lying on the ground, Shinji was nearly attacked by a monster that emerged from Mirror World, Dragreder.
However, the card deck not only protected him but allowed him to sense things when he feels that Reiko is in danger, ending up in the Mirror World where he encounters another monster, Dispider. He tries to escape the monster as it overwhelms him until he is saved by Kamen Rider Knight who defeats the monster before he and Shinji are ambushed by Dragreder, with the two barely able to get back to the real world in time.
Once coming to, Shinji finds himself in the Atori Cafe where he formally meets Yui Kanzaki and Knight's human identity, Ren Akiyama, who tries to take the Ryuki deck from him with Yui explaining to him of the Mirror Monsters and that he's targeted by Dragreder. After understanding Reiko's reasons for investigating the missing person's case, only to run into Ren who is following him to get Dragreder before Yui contacts him about the reformed Dispider.
Following Ren, and seeing the daughter of a woman who was devoured by the monster, Shinji learns that he can fight once he forms a contract with a Mirror Monster. Though warned that he'll be beyond the point of return if he commits to it, Shinji uses the Contact card Yui gives him to form his pact with Dragreder, becoming Kamen Rider Ryuki to save Knight and destroy Dispider with Dragreder feeding on the monster's essence. But once the fight ends, Ryuki is suddenly attacked by Knight, who beats him until Yui indirectly interferes and threatens to leave him.
Indebted to Ren for 30,000 yen after he paid for Shinji for the glass window Yui shattered to get their attention, Shinji later runs an errand for his boss, where Shinji encounters Shiro Kanzaki, who tells him to fight the other Riders, before finding Ren while he was hunting for a Megazelle incognito. The next day, Shinji visits Yui and recognizes the picture of her brother, believing that the Vent Decks were made to protect people from the Mirror World monsters, revealing that to be his reason for becoming a Kamen Rider.
After destroying a Gigazelle, Shinji starts to question Ren's methods after seeing Darkwing scaring a girl, unaware that she was being attacked by another Gigazelle. Later, after getting kicked out of his apartment as his rent was overdue, Shinji moves into ORE temporarily, causing much inconvenience to both himself and the company.
The next day, after a tussle with Ren and seeing that he might have a reason of his own to fight, Shinji is questioned if his resolve is strong enough to take a life, with Ren reminding him of the debt and it would be a shame if he can't pay back in full if dead. After which the two defeat a Megazelle and a Gigazelle.
But the fact that Riders must fight each other worries Shinji, remembering the ring he found on Ren during their tussle. He later learns that Reiko is hospitalized while investigating an antique shop due after finding a link between it and the missing person case, meeting detective Masashi Sudo before Reiko assigns him to continue her investigation on the owner Tomoyuki Kaga. While on his way to the store, he sensed a woman attacked by the Mirror Monster Volcancer, saving the woman and fighting the monster until Scissors arrives, with Ren later showing up to even the odds as Scissors retreats.
Believing that Scissors is Kaga, Shinji attempts to reason with the Rider, leaving his calling card to contacted. But Shinji falls for Sudou's trap, tricked into revealing Knight's identity as well as kidnap Yui. After barely enduring Scissors attack on him, Shinji learns that Kaga is not Scissors when his corpse is found by Ren who investigated the shop and finding Kaga plastered behind a wall. He later he meets up with Ren while he's chasing down Yui and her kidnapper, following him to find out that Scissors is Sudou, getting scolded for endangering Ren and Yui in his ideology.
Confronting Sudou, Shinji was about to fight him because of his evil actions until Ren arrives to settle things with Sudou. Shinji was forced to watch the two fight until Scissors is eaten alive by Volcancer, and he destroys Volcancer with his Final Vent, though he kept his ideals by not having Dragreder absorb the Mirror Monster's essence as Darkwing takes it while Knight explains there are more Riders left to defeat.
Later, while assisting Reiko in the interview a man named Togukawa Katsuhikaku on a golden crayfish, Shinji almost gets run over by a white car as he arrived in the Atori Cafe to meet Yui's grandmother who lets him move into Atori Cafe, encountering the driver Shuichi Kitaoka and grabbing him as before contacting the Sakura Company's thugs, unaware their identity and intent as he saves Kitaoka before escaping with him, disgusted by Kitaoka's methods as a lawyer. Though cornered, Kitaoka and Shinji were saved by the lawyer's apprentice Goro Yura as Shinji senses two Zebraskulls and battles one until it escaped, seeing the other destroyed by Kamen Rider Zolda.
Later, when Reiko believes that the river was polluted by Kawazu Pharmacy, resulting in both the crayfish and the haired-frog Toguwawa found, Shinji accompanies her to see the Pharmacy's representing lawyer, ironically which was Kitaoka. Soon after spending time with Kitaoka, Shinji spends the next day with the lawyer to write an article about him until he hit a nerve and was ordered to leave. Sensing the Zebraskull from before as it devours Togukawa, whose animals turned out to be fakes, Ryuki battles it until he finally destroys it.
But by that time, Zolda gets the drop on Ryuki who forced to fight him before he is forced to retreat. The next day, Shinji learns that Shimada is kidnapped last night and is sent to give the kidnapper the ransom, only to be attacked by him. However, the kidnapper is devoured by a Wildboarder and Shinji is arrested, forced to stay in jail as he can't explain what happened to the kidnapper without exposing himself and Shimada has no memory of her kidnapper.
However, when Reiko and Ren manage to get Kitaoka as his lawyer, he intended to incriminate him after learning that Shinji's actually a Rider. After his friends fire Kitaka, Shinji is freed thanks to the kidnapper's daughter Yukari, meeting her at the hospital with Kitaoka as she defends him before he drives off. But when Yukari is targeted by the Wildboarder, Ryuki joins Knight in a two-on-one fight on the Mirror Monster before Zolda arrives to take them all out.
Though Ryuki and Knight survived, Ren suffered memory loss as a result, not remembering the recent events over the year. Though told to leave him alone, Shinji follows Ren as he attempts to recover his memory, encountering every single person Ren had ever offended before he senses Zenobiter and is forced leave Ren to fight the monster as Ren regains some of his memories. But once he kills the Mirror Monster, Shinji follows Ren to Semiin University before covering for him when they encounter some enemies of Ren's. By the time he find Ren, he's with Yui with his memory completely restored as the two save the dying Professor Ejima from Terrabiter.
While investigating the Non-Stop Elevator rumor for its ties to the missing people, Shinji mistakenly believes that Zolda is Kitaoka's assistant Goro and chides him before sensing Deadlemur. But during the fight, Zolda intervenes and Ryuki was forced to fight back at the cost of seemingly killing Goro. Feeling guilty about it, Shinji quits being a Rider even if it meant that Dragreder would eat him. However, thanks to Ren exposing Kitaoka as the real Zolda, Shinji learns the truth and feels a weight lifted seeing Goro alive and resumes being a Rider with intentions of stopping the Rider War.
When asked to investigate Meirin University where rumors of masked men fighting take place, the chain of events led Shinji to meet its arrogant mastermind Jun Shibaura, who took his Dragreder card from him. But with the aid of a kindred spirit who also wants to end the war, Miyuki Tezuka, Shinji regains his card as Jun is briefly taken to jail. However the appearance of the sadistic Takeshi Asakura in the fight results in the deaths of Jun and then Tezuka. Shinji's offers of friendship eventually win Ren over, even though they also maintained something of a rivalry afterward.
At the end of the series, he was stabbed in the back by a Raydragoon before he could transform while protecting a little girl. Shinji died with a devastated Ren by his side shortly afterwards following a fight with the Raydragoons. In the end, he declared to Ren that he finally found his desire as a Rider: To close the Mirror World. Bolstered by his dying words, Ren would go on to win the Rider War, and bring Eri back to life.
Shinji's story, however, ends on a happy note. Kanzaki finally realized the futility of his single-minded obsession with his dead sister, and at her request finally ended the Rider War for good. Performing one last Time Vent, Shiro travels to the past. Instead of pictures of Monsters though, he and Yui would draw pictures of them living happily together in a beautiful world. Thus, the Rider War never happened, and the Riders lived happy new lives without needing to fight. Somehow, Shinji managed to change the Rider's fates in the end, and achieved his wish to stop the fighting.
Recent crossovers reveal that he has somehow re-obtained his Ryuki deck and Dragredder with no explanation given as to how this happened. Perhaps these are other Shinjis of different worlds, or his appearances are due to time paradoxes. Whatever the reason, Shinji Kido would always be there, fulfilling his only desire of protecting others as Kamen Rider Ryuki.
In the 13 Riders Special which is also an alternate retelling of the story, his role in the Rider War was the same, but his origin as Ryuki was much different. It was by fate he was dragged into Mirror World by a monster to be saved by the previous Ryuki, Kōichi Sakakibara. But mortally wounded and wishing to save his life, Sakakibara gave Shinji his deck and faded while Shinji became the new Kamen Rider Ryuki.
Shinji still tried to prevent the other Riders from fighting as his way to thank Sakakibara. He was lured into the Mirror World by the others under the guise of a truce, but was ambushed. While fighting the other Riders, the Ryuki deck was destroyed by Kamen Rider Odin. After Ren was killed by Kamen Rider Verde, Ren gave Shinji the Knight deck and he took the mantle of Kamen Rider Knight. Shinji promised Ren that he will fight and in one ending he takes on the rest of the other Riders.
In the other ending, Shinji destroyed the master mirror with the monsters and Riders fading away only for the Riders to reappear and Shinji regaining the Ryuki deck.
In the movie: Episode Final which is an alternate retelling of the story, Shinji continued trying to stop the battle despite Shiro's rule to fight. He appears to be deceived by Miho Kirishima while maintaining a relation to her. Later, Shinji also finds out that Yui had already met him and he feels that it was because of him that Yui is dying. Shinji also soon finds his mirror doppelgänger Kamen Rider Ryuga, where the latter tricks him into letting him absorb him under the assumption that it would save Yui. But Shinji breaks free and defeats Ryuga before he and Knight assume Survive form to defeat the horde of Hydragoons.
In this alternate retelling of the story, Shinji was revealed to have a twin older brother. Although the two looked exactly the same, they had different personalities; Shinji is good hearted and friendly, but naive, while Shinichi was more the rough kind. Nevertheless, they got along very well and were really close. However, one day, when a thug pointed his knife to Shinji, he died while protecting his younger brother. It turned out, though, that the thug was a person whom Shinichi fought before and was after him in the first place, and just got them mixed up due to his similarities.
Shinji is one of the last surviving Riders in this version of the story. He challenges Asakura to a fight, Asakura using the Survive Shippu card he stole from Ren. Later, Asakura uses the Survive deck's Unite Vent card along with all the Advent cards he's collected from the Riders he defeated, combining all the monsters (along with Dragreder) into Genosurviver.
But because Asakura had not contracted Dragreder, its head starts biting at Venoviper's head, causing Genosurviver to run amok and attempt to destroy Mirror World. Kitaoka kills Asakura and Genosurviver with his Final Vent, but he is also caught up in the explosion and dies as well.
Shinji eventually stumbles across Ren under some rubble, a direct consequence of Ren's previous fight with Asakura. At that moment, Shinji is possessed by Shinichi's spirit, causing his deck, armor and Contract Monster to turn black.
Ren shows up, revealing the Knight under the rubble was a Trick Vent-created copy. Since they are the last remaining Riders, they fight with Shinji as the winner. Odin, who serves to grant the winner's wish in this story, appears and grants Shinji's wish: to revive all the victims of the Rider War.
Kamen Rider Ryuki: World Of If
In this S.I.C. alternate retelling of the story, the remaining Riders are Ryuki (now Ryuga), Knight, Tiger, Ouja and Odin.
After Shinji becomes Ryuga, he starts to turn into a mindless rampant in the Mirror World. Tiger shows up and tries to stop him only to be easily defeated by Ryuga in his Survive mode.
Hideyuki Kagawa, as Alternative Zero shows up with an army of twelve Alternatives in order to destroy Ryuga. As they prepare to attack Ryuga, Yui Kanzaki shows up. Kagawa laughs; Yui is the source of all their troubles and if she dies everything will end. They attack her instead, but Ryuga, having his sanity returned shields her. He reverts to Shinji and dies.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga