Hajime Saito is a fictional character who appears as a primary figure in the Bakumatsu Crisis storyline. He is a tall man with a lean but fit frame, characterized by a long face, pronounced cheekbones, and narrow, piercing yellow eyes. His short black hair is neatly slicked back, with the exception of four distinctive bangs that fall over his forehead, contributing to a wolf-like appearance. During the Bakumatsu period, he was a recognizable figure as the captain of the third unit of the Shinsengumi, the special police force loyal to the Tokugawa shogunate, and he typically wore the group's traditional uniform with a long ponytail.

In terms of personality, Saito is defined by his stoic and cold demeanor. He lives by a rigid and ruthless personal code known as Aku Soku Zan, which translates to Swift Death to Evil. This philosophy drives his actions, making him a cynical and pragmatic individual who shows little regard for human life when it stands in the way of his duty. He possesses a morbid, sometimes arrogant sense of humor and can be sadistic in his methods. Despite his chilling exterior, his core motivation is the establishment and maintenance of peace and order in society, even if that society is now led by the former enemies of the shogunate. He is highly analytical, intelligent, and a keen observer of psychological weaknesses, which he readily exploits to dismantle his opponents' strategies.

Saito's role in the story is largely defined by his long-standing and intense rivalry with the protagonist, Himura Kenshin, which began during the Bakumatsu when they fought on opposite sides of the conflict. Their duels often end inconclusively, and Saito harbors a deep, lasting enmity toward Kenshin. However, this rivalry is complex; he gradually comes to respect Kenshin's unwavering pacifist vow not to kill, even if he does not personally subscribe to it. His key relationships include a grudging rapport with Sagara Sanosuke, whose combat skills he acknowledges despite their persistent friction. Within the Shinsengumi, he was a close associate of fellow captain Okita Soji. After the war, taking on the alias Fujita Goro, he works as a spy and police officer for the Meiji government, a position that forces him into uneasy alliances with his former enemies, including Kenshin, to combat greater threats to the nation's stability.

The character does undergo a subtle development over time. Initially presented as a pure antagonist intent on testing and possibly killing Kenshin, he evolves into a cynical ally. Through his collaborations with Kenshin and other members of the group, he begins to show a glimmer of acknowledgment for Kenshin's new path. Ultimately, he decides to put an end to their long rivalry, refusing a final duel because he recognizes that Kenshin is no longer the manslayer he once swore to destroy. This reflects that Kenshin's influence has, to a small degree, softened his own rigid worldview, even as he continues to carry out his duties in the shadows.

Saito is a master swordsman of the highest caliber, whose notable abilities make him one of the most formidable fighters. He specializes in a powerful left-handed thrusting technique derived from Shinsengumi swordsmanship called the Gatotsu, which has several variations for different combat situations. Beyond his swordplay, he is extremely well-versed in hand-to-hand combat, integrating techniques from martial arts like jujutsu, judo, and aikido for counters and throws. His physical strength is immense, allowing him to overpower opponents even while unarmed. He is also extremely adaptable, known to use everyday objects like belts as weapons if disarmed, and his keen analytical mind allows him to predict and exploit his opponent's weaknesses with surgical precision.