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Masato Kusaka, also known as Kamen Rider Kaixa, is the primary user of the Kaixa Gear and the major character of Kamen Rider 555. Despite being one of the main characters, Kusaka can hardly be considered a hero due to his manipulative and sociopathic tendencies, as well as his hatred of all Orphnochs, and is only allied with the protagonists due to their shared opposition to Smart Brain.
He is portrayed by Kohei Murakami who later portrayed Bud/Zyuoh Bird in Doubutsu Sentai Zyuohger.
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.
Kamen Rider Zi-O
Kusaka is somehow revived, alongside his rival Takumi as a result of a time paradox caused by the creation of Another Faiz.
Unlike his previous incarnations, Kusaka does much greater good, such as repaying someone who are related to Ryusei School and similar suffering as Masato was and risk his life to do this deadly task no matter what.
Though he acts polite towards everyone (except for Takumi), Masato Kusaka's true personality is that of a manipulative sociopath who only cares for himself and Mari. Even in regards to Mari, Kusaka only sees her as his possession and not as a human being, holding an extremely twisted "love" for her and even going so far as to rape her in the non-canon novel adaptation.
Despite this, Kusaka is extremely charismatic and excels at getting people to like and trust him. Even after his death, Mari and Keitaro still didn't realize the full extent of his sociopathy. Kusaka is also a cunning manipulator, skillfully playing people against each other such as when he ruined the friendship between Takumi and Yuji Kiba.
His most defining trait is his hatred for Orphnochs, it is so fanatical beyond belief to the point he even sees other benign Orphnochs (such as the aforementioned Yuji Kiba or Takumi after he revealed to be one) as his prey.
Kamen Rider Zi-O is the only instance where Kusaka is not entirely bereft of sympathetic qualities. Due to the time paradox caused by the creation of Another Faiz, along with the said Another Rider trying to keep his significant other (Karin Yamabuki) alive - as both of them were his classmates in the Ryusei School, he is more willing kill the said girl out of mercy to relieve her of the life imposed to her by Another Faiz. However, this doesn't change his hostile relationship between him and Takumi.
So far, Kusaka is the only Secondary Rider in the Heisei Era to never befriend with the Primary Rider and only remained hostile towards them throughout his respective series. As the Second Rider tradition start off as anti-heroes before warming up to the main character(s).
A internet meme holiday called "Kaixa Day" is celebrated every September 13 - the date is a play on Kaixa's transformation keycode [9-1-3].