Saitō Hajime stands tall with a lean, fit frame. His long face bears pronounced cheekbones and tear-troughs, narrow piercing yellow eyes, and short black hair slicked back save for four distinctive bangs over his forehead. Often described as wolf-like, he wore a traditional Shinsengumi uniform with a long ponytail during the Bakumatsu, later adopting a blue policeman's uniform and short hair in the Meiji era.

Stoic and calm, he ruthlessly embodies the Shinsengumi code "Aku Soku Zan" (Swift Death to Evil). His justice is cynical and pragmatic, showing scant regard for human life and occasional morbid humor. Despite his cold exterior, he dedicates himself to maintaining peace and order, even under a government of former enemies. Highly analytical and observant, he manipulates opponents by exploiting psychological weaknesses. Married to Tokio, he adopted a son named Mishima Eiji but expresses annoyance when domestic life intrudes on his professional image.

As former captain of the Shinsengumi's third unit, he served the Tokugawa shogunate during the Bakumatsu, participating in pivotal events like the Ikedaya incident to prevent Kyoto's burning. After the shogunate's defeat, he vanished into hiding before resurfacing as Fujita Gorō, a Meiji government spy and police officer conducting covert operations, intelligence gathering, and eliminating societal threats.

His relationships pivot on rivalry and reluctant alliances. He harbors deep enmity toward Himura Kenshin from their opposing Bakumatsu loyalties; their clashes often end inconclusively, though he gradually respects Kenshin's pacifist vow. With Sagara Sanosuke, he shares a grudging rapport, acknowledging his combat skills despite friction. Within the Shinsengumi, he closely associated with Okita Sōji and Nagakura Shinpachi, commanding forces at the Battle of Aizu after Hijikata Toshizō's injury.

A master swordsman, he specializes in the Shinsengumi-derived Gatotsu technique—a powerful ambidextrous thrust to disorient opponents. His style blends kenjutsu with hand-to-hand combat, incorporating jujutsu, judo, and aikidō for counters and throws. Exceptionally strong, he overpowers Sanosuke unarmed, and adapts tactically by weaponizing everyday objects like belts when disarmed. Keen observation lets him dismantle strategies and exploit vulnerabilities.

During the Kyoto Arc, he tests Kenshin under government orders before collaborating against Shishio Makoto, eliminating adversaries like Uonuma Usui while surviving grave injuries. In the Jinchu Arc, he aids Kenshin against Yukishiro Enishi, defeating Yatsume Mumyōi but refusing to kill him—reflecting Kenshin's subtle influence. He ultimately prioritizes police duties over his rivalry, transferring posts before investigating threats in Hokkaido.

In the Bakumatsu series, as a Shinsengumi member, he confronts anomalies like a chronometer manifesting in Kyoto and assists evacuating civilians during supernatural fires, maintaining composure amid chaos.