Souji Okita's profound childhood trauma shaped his complexity. Orphaned early, his sister Mitsu left him at the Shieikan dojo around age nine, where severe bullying from older students ensued. This isolation fostered emotional detachment and a conviction of his own worthlessness, driving him solely to pursue strength and prove independence. Kondou Isami's intervention offered limited solace, forging a deep but exclusive bond that hindered Okita's broader emotional growth. His worldview centered entirely on becoming Kondou's useful tool, neglecting typical social understanding or moral development.

Within the Shinsengumi, Okita served as Captain of the First Unit, earning the epithet "Sword of the Shinsengumi" for exceptional swordsmanship. He mastered Tennen Rishin-ryū young and developed the lethal Sandanzuki technique (also called Mumyō-ken or Hyakkidō-ri), capable of simultaneously striking an opponent's neck and shoulders. His demeanor blended carefree, often sadistic teasing with ruthless combat efficiency. He openly threatened violence, viewing Chizuru Yukimura upon her arrival as a liability to eliminate if she hindered the Shinsengumi. This reflected his fractured moral compass, where life held value only in relation to the group's—specifically Kondou's—needs. His relationships mirrored this perspective: intense jealousy over Kondou's attention directed at Hijikata Toshizō, a mutually respectful but distant rapport with the similarly reserved Saitō Hajime, and antagonistic teasing of Hijikata stemming from envy.

Okita's character evolved significantly, largely catalyzed by Chizuru Yukimura. His relentless teasing of her initially provided amusement or provoked reactions. Following the Sanjo Ohashi incident involving her brother Kaoru, his behavior subtly softened. He expressed genuine concern for her safety, cautioning against recklessness and later protecting her from threats like Kazama Chikage, even while weakened. Chizuru consistently treating him as an equal, not a weapon or child to shield, gradually eroded his emotional barriers. This shift intensified during pivotal Shinsengumi crises like the departure of members forming the Goryo Eji. Okita displayed uncharacteristic vulnerability when distracted by Saitō's absence, masking distress behind declarations of duty while his voice and gaze betrayed reluctance to fight former comrades.

His physical decline from tuberculosis became a central struggle. Historically believed to have developed around the Ikedaya incident or later, the disease progressively weakened him, causing collapse during battles and confinement. Desperate to remain useful to Kondou, he succumbed to Kaoru Nagumo's manipulation and consumed the Ochimizu (Water of Life). This transformed him into a Rasetsu (Fury), granting enhanced strength and regeneration at the cost of bloodlust and eventual corruption. While the elixir allowed him to fight, it accelerated his physical deterioration. In some story paths, driven by rage after Kondou's injury or death, he embraced his role as a mindless "sword," leading to self-destructive rampages halted only by Chizuru's intervention. His Rasetsu state culminated in different fates: succumbing to tuberculosis historically, or sacrificing himself in a final battle to protect Hijikata, his body disintegrating into dust after annihilating enemies.

The spin-off *Hakuouki: SSL* presents an alternate, healthier incarnation within a modern high school setting. Free from tuberculosis and warfare, this Okita retains sharp wit and mischievous teasing, particularly toward Hijikata (his classical literature teacher). He excels academically and in kendo, earning a collegiate scholarship. While still seeking Kondou's (now the principal) approval, his emotional maturity significantly improves. His relationship with Chizuru develops into a more typical teenage romance, marked by mutual care without historical trauma, though he continues to enjoy provoking others.