Within the Rurouni Kenshin series, Hajime Saitō is a former captain of the third unit of the Shinsengumi, a pro-shogunate special police force active during the Bakumatsu period. Following the defeat of the shogunate forces and the subsequent Meiji Restoration, he goes into hiding and later resurfaces under the alias Fujita Gorō, working as a spy and police officer for the new Meiji government. His character is defined by the Shinsengumi code Aku Soku Zan, which translates to Swift Death to Evil. This philosophy is his primary motivation, driving him to mercilessly eliminate anyone he deems corrupt, despotic, or a threat to the peace and order of the new era, even if that society was built by his former enemies. He is portrayed as morbid, arrogant, and possessing little compassion for his foes, yet these traits are balanced by his abilities as a superb investigator, a quick analyst, and a formidable fighter.

Saitō's most significant relationship is his long-standing rivalry with the series' protagonist, Himura Kenshin, a former assassin for the imperialist cause. Their history of unresolved duels from the Bakumatsu carries into the Meiji era, where their uneasy alliance forms a core part of the narrative. Although he joins forces with Kenshin to confront greater threats like the revolutionary Shishio Makoto, Saitō never considers the former manslayer an ally, viewing him instead as an adversary whose vow to never kill again he finds weak and contemptible. After Kenshin proves his resolve, Saitō decides to end their personal rivalry, though he maintains a pragmatic working relationship with him for the sake of the government's missions. Other key relationships include his unseen wife, Tokio, to whom he is married, and a brief but notable fistfight with the brawler Sagara Sanosuke, whom he defeats with ease, demonstrating his physical strength and skill in hand-to-hand combat.

As a combatant, Saitō is exceptionally powerful. His signature technique is the Gatotsu, a unique and deadly left-handed thrust derived from the Shinsengumi's Hirazuki style. He has mastered several variations of this single attack, including a standard shoulder-level thrust (Ishiki), a downward strike (Nishiki), an upward strike (Sanshiki), and the point-blank Zero Stance (Zeroshiki) which uses immense upper body strength. Despite relying heavily on one technique, its many applications and the raw speed and power behind it make it a formidable weapon. The motion of the Gatotsu can even be performed without a sword, using his bare hands. Beyond swordsmanship, Saitō is also highly proficient in unarmed combat, utilizing a fighting style that resembles boxing with excellent footwork, speed, and defensive capabilities. He is also an extremely analytical fighter, able to quickly exploit an opponent's weaknesses, which makes him a deadly and efficient agent for the Meiji government.