Masato Kusaka is the wielder of the Kaixa Gear, which he uses to transform into Kamen Rider Kaixa and the tritagonist and main anti-hero of Kamen Rider 555.
He is a member of Ryusei School who was apparently absent from the reunion, but signed the autograph card. He is the only Ryuseiji who was capable of wearing the Kaixa Gear without disintegrating afterwards and as such is the Gear's primary wearer. As a child, Masato was often bullied and alone with only Mari Sonoda as his friend. Through his childhood, he falls in love with Mari, though his feelings were not reciprocated back.
As children, Mari often saved him from bullies and as a result he has fallen in love with her and is willing to go to elaborate lengths to eliminate any obstacles between him and her. Jealous of Takumi's closeness with Mari and seeing him as an obstacle, he will stop at nothing to damage Takumi's standing, and as a skilled manipulator, he often secretly plays Takumi and Kiba against each other so Takumi will have no allies. Also to be noted is that he seemingly considered destroying Smart Brain more important than Mari. He also passionately despises the Orphnochs, regardless of whether they "have souls" or not and sweared to destroy them all. His hatred was driven by the events that took place at the Ryusei School reunion and he continually wipes his hands to remove the imagined blood he remembers from that night. Like some of the other Secondary Riders, Kusaka acted as a mean and nasty person towards the Main Rider but unlike the rest of them, he never reformed and befriend said Rider. While this is somewhat understandable due to his past, it should be noted that Takumi tried to understand his pain and help him despite being treated cruelly by Kusaka thus mostly making this moot. It is also notable that Kusaka spent a great deal wallowing in self pity while stating that others such as Takumi wouldn`t understand his pain despite Takumi trying to understand said pain showing hypocrisy. Despite being mostly unlikable throughout the series, it seems Kusaka somewhat wanted to stop the Orphnochs to save humanity although most of his reasons seemed to be for revenge.
In Zi-O portrays Masato a much positive than his previous appearance, where he willingly to sacrifice his life on behalf of his fellow Ryusei alumnis.
As stated above, Kusaka had a history with the Orphnochs which fueled his hatred for them. While originally acting friendly towards Takumi, he eventually showed his true colors and his motives were revealed.
Masato Kusaka is the only one to have worn all three of the Kamen Rider belts (Faiz, Kaixa and Delta). He wore the Faiz Gear to trick Yuji Kiba. He wore the Delta Gear to battle the Beetle Orphnochs (Rhinoceros and Stag).
Masato had a great deal more Orphnoch DNA implanted into him, allowing him to use the Kaixa gear safely for a time, but near the end of the series, it began to wear off, and using the Kaixa Gear was becoming increasingly harmful to him. In the end, his neck was snapped by Yuji Kiba, who claimed the Kaixa Gear as his own towards the end of the series and dies after all of his Orphnoch DNA ran out causing himself to disintegrate into dust with Takumi and the others being unaware of his death until later when his picture of a young Mari was found. This caused Takumi`s anger to intensify as he cursed all Orphnochs including himself.
Kamen Rider Zi-O
When timeline was altered by the results of one of Ryusei’s alumni Sakuma turned Another Faiz and Fourze, Masato was revived. When learning Sakuma revived his deceased girlfriend Karin Yamabuki, who supposedly died by car crash in original timeline, and now an Amanogawa student by stealing the high school girls who has a same birthday as her, it was Karin who requested Masato kill her, to stop Sakuma’s killing spree, though caused both Takumi, and the Time Riders Sougo Tokiwa and Geiz Myoukouin misunderstood the situations, until the three latter learns why Karin hasn’t aged for 15 years in her pictures between Ryusei and Amamanogawa. Takumi eventually saved his life from having his death repeated again, no matter how much Masato treated him badly, with both former Kamen Riders witnessed Sakuma’s redemption after being the former 2-in-1 Another Rider defeated by Zi-O and Geiz in both timelines.
In the alternate reality movie Kamen Rider 555: Paradise Lost, Masato was killed by Leo. Keitaro Kikuchi briefly wore the Kaixa Gear by drinking the experimental transformation elixir, 'Transform 1-Shot' (変身一発 Henshin Ippatsu) and killed the Lion Orphnoch. Due to the elixir, Keitaro was unharmed. However, when the formula ran out, the Kaixa Gear disintegrated instead.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Stronger
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga