Tachibana was originally a researcher of the Humanity Foundation who graduated from the Sazahaibu University and was selected to be the user of Rider System 01 after Go Kiryu was found to be incompatible with the system. By the start of the series, Tachibana begins to suffer a form of cellular breakdown with only sealing the Undead slowing down the process. That, combined with his feelings towards Kenzaki being his backup when he can't manage it, Tachibana blames Karasuma for his condition. When he attempted in vain to seal the Locust Undead, Tachibana is forced to abduct Karasuma just prior to the Undead attacking BOARD. Though he intended to get the answers he needs, Karasuma was in a coma and put on life support with Tachibana safeguarding him until Hirose finds him. Though he leaves Karasuma in her care, it only revealed that the "Karasuma" he safeguarded was only a decoy. After his doctor and dear friend Sayoko Fukasawa relays Karasuma's message, that his subconscious fear is causing the breakdown, Tachibana encounters Isaka, the Peacock Undead, who personally attempts to retrieve him for his Leangle project. After escaping Isaka, Kotaro manages to talk Tachibana not to let his fear control him though Tachibana denies that. He arrives to save Blade from the Trilobite Undead, however Garren's fusion rate goes down during his fight with the Undead. After being saved, Tachibana begs Karasuma for a means to be free of his fear, but Karasuma say he can't.
Though Tachibana attempts to resume a normal life by dating Sayoko, the Zebra Undead forces him to reveal himself as a Rider in front to Sayoko before vainly fighting the Undead before Blade arrives to aid him. In spite of Sayoko asking him to go off with her, Tachibana turns her down. While at BOARD's original, Tachibana vainly battles the Peacock Undead, who defeats Garren and takes him with intend to have Tachibana work for him by removing him of his fears with exposure to Schuld Kestner seaweed, an extinct plant that intensifies the aggressive nature of the subject. Though he became more aggressive and worked under Isaka for more of the seaweed, Tachibana was unaware of the eventual damage it would do to his nervous system. Though Sayoko warned him in vain, Tachibana shrugged it off until he learned that Isaka murdered Sayoko for meddling in his affairs, defeating the Peacock Undead and sealed him. Later, as his way of repenting for being manipulated by an Undead, Tachibana takes Mutsuki under his wing.
During the final few episodes, Kamen Rider Garren came face to face with the Category King of Diamonds and battled it out with him. The Undead told him that if he was sealed, Joker would be declared winner and that all of humanity would suffer because of his choice. Garren discarded this and answered, "I will believe in my friends." The battle ended with Garren nowhere to be found and the King Undead sealed leaving the Joker as the winner however, it would cause an unintentional death of billion species due to Joker's uncontrollable Undead nature. He is found upon the arrival of the Chief telling Kenzaki that he was saved because of him. He arrived assist an unarmed Blade King Form, finishes the remaining Black Roach Undeads with King Rouzer, and wished him good luck against Hajime. Due to his Rider system is still broken, Tachibana fought a multiple Black Roach Undeads alone as a normal person while waiting for Kenzaki stops Hajime's suffering. After all Black Roach Undeads are suddenly disappear, he and Mutsuki went to the forest where Kenzaki and Hajime fought, only finds Hajime alone who's try to follow Kenzaki but lost track of him, soon realized that Kenzaki has becoming an Undead and disappeared from battling Hajime in person again. After Kenzaki's disappearance, Tachibana sees a photo of Kenzaki together with him and remember their good times, prior becoming an Undead.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga