Masato Kusaka is a member of the Ryusei School alumni group and serves as the primary user of the Kaixa Gear, transforming into Kamen Rider Kaixa. His past is defined by a traumatic encounter with the Dragon Orphnoch, who killed him and later revived him with a large amount of Orphnoch DNA. This event gave him the ability to safely wear the Kaixa Gear for a time, but it also created an intense and personal hatred for all Orphnochs, which drives much of his behavior throughout the story.

In terms of personality, Kusaka initially presents himself as a friendly and capable ally, but he gradually reveals a manipulative, possessive, and deeply conflicted nature. He craves affection and belonging, particularly from his childhood friend Mari Sonoda, yet his methods are often controlling and destructive. His motivations stem from a desire to be accepted and to protect Mari, but his approach is shaped by paranoia, jealousy, and a refusal to trust others, especially Takumi Inui and Yuji Kiba. He sees himself as a hero, but his actions frequently put him at odds with the other protagonists, making him an antagonistic force within the group.

His role in the story is complex. As the wearer of the Kaixa Gear, he is a second Rider who fights Orphnochs, but he is often in direct conflict with Takumi Inui, the main Rider, and Yuji Kiba. He attempts to manipulate events from behind the scenes, creating distrust among the main characters and sabotaging their alliances. Despite his antagonistic role, he is crucial to the narrative tension, as his presence forces the other characters to confront their own motivations and relationships. He also briefly wields the Faiz Gear to deceive Yuji and later uses the Delta Gear in battle against the Beetle Orphnochs, making him the only character in the series to use all three Rider belts.

Key relationships define his development. His fixation on Mari Sonoda is central to his character; he views her as a source of the maternal affection he lacked and becomes jealous of anyone close to her, particularly Takumi. His rivalry with Takumi is bitter and personal, as Kusaka resents Takumi for his connection to Mari and his ability to use the Faiz Gear without apparent effort. His relationship with Yuji Kiba is equally hostile, as Yuji represents the kind of Orphnoch that Kusaka despises, yet Yuji also eventually becomes the one who ends Kusaka's life. Throughout the series, Kusaka's attachments are fragile and often built on deception, leading to his increasing isolation.

His development follows a tragic arc. As the series progresses, the Orphnoch DNA implanted in him begins to degrade, making the transformation to Kaixa physically harmful and eventually life-threatening. This physical deterioration parallels his emotional unraveling, as his schemes and manipulations collapse and he loses the trust of everyone around him. In the end, Yuji Kiba kills him by snapping his neck, and without the sustaining Orphnoch DNA, Kusaka's body disintegrates into dust. His death is discovered later through a photograph of young Mari, and despite his antagonistic role, Takumi shows sorrow for him, recognizing that Kusaka was also a victim of his circumstances.

As Kamen Rider Kaixa, Kusaka possesses notable abilities. The Kaixa Gear outputs more Photon Blood than the Faiz Gear, making Kaixa physically stronger, though slightly slower. He wields the Kaixa Shot for powerful punches, the Kaixa Pointer for dive kicks, and the Kaixa Blaygun, which has both a gun mode and a blade mode. His finishing moves include the Gold Smash kick and the Kaixa Slash. He also displays a characteristic habit of adjusting his neck collar after transforming.