Masato Kusaka

Masato Kusaka aka Kamen Rider Kaixa is the primary user of the Kaixa Gear and the deuteragonist of Kamen Rider 555. Despite being one of the main characters, he can hardly be considered a hero due to his manipulative and sociopathic tendencies, as well as his hatred of all Orphnochs, and is only really allied with the protagonists due to their shared opposition to Smart Brain.

Kamen Rider 555
Masato Kusaka grew up as one of the orphans in the Ryusei School fostered by Hanagata, the former head of Smart Brain. Kusaka was among the children implanted with Orphnoch DNA to grant them use of the Kaixa Gear though Kusaka was implanted with a great deal more DNA than the others, allowing him to transform and use the Kaixa Gear without dying afterwards.

Growing up, Kusaka's only friend was Mari Sonoda, leading to him forming an unhealthy attachment to her and eventually coming to see her as his possession. After the massacre at the Ryusei School reunion was instigated by the Dragon Orphnoch, Kusaka came to see all Orphnochs as monsters and vowed to wipe the all out.

Showing up to claim the Kaixa Gear, Kusaka becomes its primary user after he is the only one to survive transforming with it. Though he gets along well with Mari and Keitaro, Kusaka does not get along at all with Takumi, who is the only one who can see him for the manipulative sociopath he really is.

Eventually, Kusaka's Orphnoch DNA starts to run out and he dies after Yuji Kiba steals the Kaixa Gear from him and snaps his neck, disintegrating once all his Orphnoch DNA disappears and leaving behind his picture of a young Mari for her and Takumi to find.

Kamen Rider 555: Paradise Lost
In the alternate reality movie Kamen Rider 555: Paradise Lost, Kusaka was killed by Leo during the Riotrooper raid on the human sanctuary "Paradise". His Kaixa Gear was later used by Keitaro after he drank a transformation elixir allowing him to use it. The Gear disintegrated once the elixir ran out.

Kamen Rider 555: Grotesque Flowers
In the non-canon novel adaption Grotesque Flowers, Kusaka is the one responsible for killing Yuka Osada, a peaceful Orphnoch, driving Yuji Kiba to hate the human race. He later rapes Mari, causing her to become hopeless and depressed, and in response has his limbs ripped off by a disgusted Kiba. He is later taken in by his own admirer, Saya Kimura, before resurfacing to try and ruin Mari's life. He finally meets his end when he is killed by Takumi Inui.

OOO, Den-O, All Riders: Let's Go Kamen Riders
Kamen Rider Kaixa was part of a group of secondary riders led by Akira Date/Kamen Rider Birth who arrived to reinforce the primary riders in their fight against the Rock Great Leader. The Great Leader is finally defeated when all the Kamen Riders got on their bikes and performed the "All Rider Break" attack to destroy the Great Leader.

Heisei Rider v. Showa Rider: Kamen Rider Taisen feat. Super Sentai
Takumi is shown to have become depressed following Masato's death, which in the film is revised so Takumi witnessed him dying while fighting the Arch Orphnoch. His spirit later appears when the Badan Empire opens up the gate between the mortal world and underworld to taunt Takumi, getting him to go after Kamen Riders Wizard and X. However, the two Riders convince him to move on as Kusaka's spirit disappears once Badan's plans are foiled.