Boomerang Bigot: He had a time of wanting to kill all Orphnochs despite being one himself. This can be justified, every Orphnoch he met was either killing everyone around him, or was in their human form the entire time.
Death Seeker: When he was tricked into thinking he attacked the Ryusei School reunion, he actively tries to get Kiba to kill him.
Defrosting Ice King: At the beginning of the series, he was as rude as possible and tried to distance himself from Mari and Keitaro, only living at Keitaro's house as he was forced too. As the series goes on, he's noticeably less aggressive and more involved with his friends.
Heroic BSOD: Twice, once when he is rescued by Yuka. After he finds out that she is an Orphnoch, and questions if other Orphnoch aren't complete monsters, making him unable to fight. He gets over it, as he realizes that he should only attack those attacking humans. The second time is when he couldn't protect Mari from Sawada, causing her death. This is only further when he is forced to transform in front of his friends and have them be scared of him for a few episodes. It got worse again, when he was tricked into thinking he attacked the Ryusei School reunion, causing him to be a Death Seeker.
Meaningful Name: "Inui" contains the character for "dog" (Takumi is the Wolf Orphenoch.)
Memetic Hand Gesture: His signature wrist flick, which displays both his refined, hi-tech aesthetic and identifies him. Whether he's wearing the Faiz or Delta Gears, you'll know it's him by that little flick.
Wolf Man: Well, a humanoid wolf as the Wolf Orphnoch.
Masato Kusaka/Kamen Rider Kaixa
The primary user of the Kaixa Gear.
All Love Is Unrequited: He is love with Mari since childhood, she never knew till he flat out told her. And at the end she seemed more interested in Kiba and later Takumi. This doesn't end well for him.
Jerkass: While his predecessors and many sucessors in the Secondary Rider line are more of Jerks With Gold Hearts, he's so far the first of said category to be a full blown jerk, followed by Mitsuzane Kureshima (the latter is more of a Tertiary Rider). Curiously, Mitsuzane shares many traits with him.
Karmic Un-Death: In the Faiz novel, his stalkerish behavior towards and rape of Mari gets paid back by Kiba literally ripping him limb-from-limb and his getting "taken in" by his own Stalker with a Crush, Saya Kimura, very much in the fashion of Misery.
Trauma Conga Line: Holy. The first episode alone, has him being getting into a car accident putting him in a coma for 2 years. Then dying and reviving as an Orphnoch. He then finds, one his parents are dead, two he's homeless as his uncle sold his house, and that his uncle is corrupt. Three his girlfriend moved on and is dating his cousin, and his cousin tells him this in the most jackass way possible. Then he finds out his girlfriend was a Bitch in Sheep's Clothing the whole time.
Determinator: She has her moments. Like trying to transform to Faiz despite knowing it won't work.
Dude Magnet: She has three people after her. Kusaka and Sawada have loved her since childhood. While Kaido gets a crush on her as well. Based on extent that Takumi is willing to go for her, he probably likes her as well.