The genius swordsman and officer of the Shinsengumi. He has a carefree personality and a friendly, sometimes child-like disposition, but displays frighteningly deadly skill in combat situations. He loves sweets, and has an affinity with children, with whom he loves to play with and who adore him in return.
He is first assigned as the assistant vice captain; later in the series, he gets assigned as the captain of the first squad. He has strong bonds with Kondo, who essentially raised Okita from when he was an adolescent after his mother and siblings could no longer afford to care for him, and Hijikata, whom he loves and respects like a brother. He is the first person to discover Sei's true gender and he is her confidant on any matters relating to the fact. He'd wanted her to go back to a normal girl's life after she had gotten her revenge due to the brutality of the life of a Samurai, and he often tries again to convince her to do so whenever situations that he doesn't think she can handle arise.
In contrast to this mentality, he more often sees her as a boy, sometimes even forgetting that she is a girl, which Sei is both irritated by and grateful for. As Sei's mentor, he trains her, and is not afraid to be strict or at times, even cold towards her if he feels she needs it, but he is more often found hugging, teasing or being silly around her, and is in fact noted to be "too soft" on her by his peers. He is protective of Sei, and has concern for her well being, often worrying about what she's doing or where she is. He's always quick to rush by her side if he feels she's in danger, and he frequently serves as a form of comfort and reassurance to her. After the Ikedaya Jiken, he admits he has come to care for Sei as if she were his own kin (like he does Hijikata), but it isn't until over a year following those events that he realizes he's actually in love with her. Ironically, everyone else (except for Sei) had already assumed that he and Sei were romantically involved with each other quite some time before that. Okita's obliviousness to love stems from a traumatic incident that occurred when he was 17, when a woman he'd turned down attempted suicide in front of him. Though the woman in question survived, the incident troubled Okita deeply and he came to the conclusion that falling in love wasn't worth such pain, and consequently he kept his distance from women ever since (something he later realizes didn't occur when he met Sei, because of her nature as a girl who wanted to be thought of as a man). Another hindrance to any potential romance he may have is that he has dedicated his life to the path of the Samurai, and has long since vowed to himself that he'd never marry. Because of this, despite his feelings, he is more willing to let Sei live the life of a normal girl and even get married to someone else, than to vocalize any desire to be with her, as he doesn't see himself as one who could bring her true happiness.