He is a troubled teenage highschool boy who was selected to become Leangle by the evil will of the Spider Undead (Category Ace of Clubs). There was an instance, however, when Go Kiryu, the initial candidate to become Kamen Rider Garren, took it from him and, with the will of the Spider Undead's parasite spiders, used the system to assault Kamen Rider Blade and Kamen Rider Chalice. Although initially afraid of the dark when fighting the Mole Undead, Mutsuki overcame his fear when he saw the smiles on the peoples' faces he rescued.
Unlike Kamen Blade and Garren, the first BOARD-created Riders, he gets no Jack form mainly because of a lack of a Rouze Absorber, but unseals other Undead to aid him, especially in the battle against the Keroberos Undead. This ability was removed from him when Joker was the only remaining Undead.
Mutsuki's high-class Undead seem to be the kindest of all the high-class Royals. His Category King is a pacifist, his Category Queen has strong sense of honor and loyalty, and his Category Jack is really laid back, and sometimes lazy, only fighting to defend himself. All of them tend not to fight, unless it was necessary. And with the help of his Category King and Queen, only then he is able to suppress the power of the Spider Undead and properly seal him. Mutsuki's girlfriend Nozomi provided a significant amount of help as well; though she couldn't assist with fully suppressing the Spider Undead's power, her love for Mutsuki enabled him to at least partially resist the mind control over the course of the series.
Mutsuki was able to figure out that Kamen Rider Chalice is really the 53rd Undead: Joker Undead and that he will become the strongest enemy when the rest of the Undead are sealed. In episode 31, he says to Garren that he will defeat him before that happens. Mutsuki at one time had all of Kamen Rider Chalice's sealed cards because of the Category King Caucacus Undead (in episode 35, he uses Chalices Spinning Dance to defeat the Jellyfish Undead). He tells Mutsuki that he should stop caring for his girlfriend if he really wanted to become stronger. He continued being controlled by the darkness of the Category Ace until episode 42 when he sealed the Tiger Undead who now encourages him to fight only for the sake of others.
Eventually, when Kenzaki is not able to defeat Joker/Hajime, Mutsuki decided that it was he who would seal the Joker. He battled Joker with all his might and despite that, he lost the battle. He was last seen with his girlfriend playing some basketball.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga