Okita is usually seen wearing a blue Shinsengumi haori; unlike the rest of the members, he does not appear to wash it, and wears the bloodstains acquired on it with pride. He wears grey hakama pants with no shirt, along with a pair of tabi (socks) and waraji (straw sandals). His other outfit consists of a maroon kimono over a white nagajuban (undergarment). He wears the same hakama pants, tabi and waraji as he does while wearing his haori. Though similar in appearance to Goro Majima, he does not have any tattoos, and sports a different hairstyle, with his short hair pulled up into a high ponytail. He also wears a black beaded necklace. His eyepatch is fashioned from a leather strap and a katana's tsuba. Okita wields both a katana and a wakizashi, though he appears to favor the wakizashi.
Hirayama Goro (later known as Okita Soji) was previously a member of the Roshigumi, along with Serizawa Kamo and Hirama Jusuke. When he and Jusuke wake up to see Kamo being attacked, they rush in and kill the real Okita Soji. Kondo Isami and Hijikata Toshizo witness the attack, and propose that Hirayama, Hirama and Serizawa take on the identities of Okita Soji, Nagakura Shinpachi, and Inoue Genzaburo. The three go on to become 1st, 2nd and 6th division captains of the Shinsengumi respectively. Hijikata and Kondo are the only other members of the Shinsengumi aware of their true identities, until both Okita and Kondo tell Sakamoto Ryoma the truth of the matter.
Okita first appears after Nagakura and Ryoma's fight outside of Shinsengumi HQ. He is unimpressed with Nagakura's willingness to trust Ryoma, telling him to be "more aggressive" with the tryouts (i.e. he should have killed Ryoma), and instigates a fight of his own, throwing his sword and quickly pinning Ryoma down. When the dust settles, Nagakura points out that neither of them would have truly won, seeing as how Ryoma holds a dagger at Okita's chest.
which Ryoma commonly ignores. He is opposed to Ryoma's aversion to killing, and suspects him as a traitor against the Shinsengumi from early on. He is shown to follow Ryoma when he goes to confront Kondo, and ends up overhearing their conversation, learning of Ryoma's true identity and his goal in the Shinsengumi - to find the Tennen Rishin-ryū user that killed Toyo, his adoptive father.
Okita uses this information to stage an elaborate plan, with the end goal being to fight Ryoma one-on-one. He lies to Ryoma, telling him of a gathering of Ishin-Shishi that he plans to raid, and asks for his help in doing so; Ryoma suspects he has ulterior motives, but goes along with him anyways, and they fight alongside each other to reach Sengoku, the man that Okita had secretly conspired with to make it all happen. Okita then kills Sengoku for "talking too much" (i.e. nearly revealing their plans), which he justifies by saying he had broken an unspoken rule of the Shinsengumi.
Okita tells Ryoma that he is the Tennen Rishin-ryū user that he
has been pursuing, and instigates him further, recounting how it felt to cut Toyo down. He and Ryoma fight, and he ends up losing, telling Ryoma to kill him and get his revenge, but Ryoma refuses, realizing that Okita's fighting style is much different than the man that he had fought a year prior. Okita concedes, and reveals that he just wanted a true fight between them, with no holds barred. From this point forth, Okita begins to have more respect for Ryoma after being shown his true potential in battle.
After Inoue Genzaburo's death, Okita is determined to find the culprit, and makes it his own personal mission to do so. He refuses help from Harada Sanosuke and calls on his men to search for information. This eventually leads to yet another confrontation between Ryoma and Okita, with Okita convinced that Ryoma is the only one who is physically capable of taking down Inoue.
Ryoma protests, reminding Okita that there is another Sakamoto Ryoma that has been wreaking havoc around town, but Okita dismisses this, saying that the real Ryoma is the only one who has reason to kill Inoue, because he has been the Tennen Rishin-ryū user that Ryoma has been searching for all along.
Ryoma is shocked, not previously having known this, but Okita believes that he's "playing dumb" and refuses to answer whether or not Inoue is really the one that killed Toyo, leading them to fight once again.
Ryoma wins once again, and Okita tells him that Inoue - who he also reveals to be his mentor - was the one that killed Toyo. Kondo Isami, leader of the Shinsengumi, shows up and explains Okita's past to him.