Sougo Okita is the eighteen-year-old captain of the first division of the Shinsengumi, the police force charged with keeping order in Edo, and he is recognized as the strongest swordsman in the organization. In his youth he lost his parents and was raised by his older sister, Mitsuba, who cared for him after their loss. He later trained at Isao Kondo's dojo, where Kondo became a mentor and father-like figure, and Okita developed a lasting devotion to him. This background shapes much of his character, explaining both his protective feelings toward his sister and his fierce loyalty to the small circle of people he truly trusts.

In everyday situations Okita comes across as relaxed, childish, and fond of mischief, speaking in a flat, deadpan manner and spending much of his duty time napping under a distinctive eye mask. Behind this casual front lies a pronounced sadistic streak. He takes open pleasure in tormenting people, and his favorite target is Vice-Commander Toshirou Hijikata, whom he constantly schemes against with the stated aim of taking his position. The hostility is partly personal, since Okita viewed Hijikata as a rival for his sister's affection and resented the cold way Hijikata treated Mitsuba despite her obvious feelings for him. Even so, Okita regards Hijikata as one of his closest friends alongside Kondo and the freelance samurai Gintoki Sakata.

Okita's deeper motivations center on protecting those he cares about and maintaining peace. Beneath his flippant attitude he holds a genuine sense of duty, and he follows an unspoken principle that if any member of the bond he shares with Kondo and Hijikata strays from the right path, it is his responsibility to draw his sword against them and bring them back. His constant attempts to undermine Hijikata operate as a running rivalry, but they do not erase his underlying respect and affection for his fellow officers.

In the story Okita serves as a supporting but highly capable presence within the Shinsengumi's operations, joining major conflicts and backing up his comrades in moments of crisis. In the films he appears in this supporting capacity, and in the alternate future shown in one of the later movies he is depicted in a strikingly different form, as a wandering figure with calm speech and a reputation as a manslayer, illustrating how radically the character can shift when removed from his usual setting.

As a fighter Okita combines superb swordsmanship with unconventional tactics. He is considered the finest blade in the Shinsengumi, and his combat skill has been compared to that of Kamui, a warrior of the powerful Yato race. He is also a capable marksman and frequently relies on a bazooka and other heavy weapons rather than his sword. His agility and clever battlefield thinking make him a difficult opponent, although his physical endurance is relatively low, leaving him vulnerable when he does take a direct hit.