Hajime Saitō is a former captain of the third unit of the Shinsengumi, the pro-shogunate special police force active during the Bakumatsu period. Following the Meiji Restoration, he works as a special agent for the Meiji Government's Department of Internal Affairs, often operating under the alias Fujita Gorō. He is a long-standing rival of the series' protagonist, Himura Kenshin, a rivalry that dates back to their days on opposite sides of the conflict.

Saitō is defined by his unwavering commitment to the Shinsengumi code of Aku Soku Zan, which translates to "Swift Death to Evil." This philosophy dictates that evil must be identified and eliminated immediately and without mercy. He is portrayed as a morbid, arrogant, and ruthless individual who shows little regard for human life and has even admitted to having an inclination to kill. Despite these harsh qualities, he is a superb investigator and a highly disciplined fighter who expects those serving in military or police roles to perform their duties without allowing personal feelings to interfere. He believes in maintaining peace and order, even in the new society created by his former enemies, and is willing to perform the necessary "dirty work" by killing those he considers corrupt, despotic, or beyond saving.

Although normally serious, Saitō possesses a morbid and slightly sadistic sense of humor. His personality serves as a cynical foil to Kenshin's idealistic vow never to kill again. While Saitō respects Kenshin's strength, he views him more as an adversary than an ally. During the conflict with the former assassin Shishio Makoto, Saitō joins forces with Kenshin to combat a greater threat to the nation, forming an uneasy and pragmatic alliance. Later, after acknowledging the strength of Kenshin's resolve to protect without killing, Saitō decides to put an end to their personal rivalry.

Physically, Saitō is a tall man with a lean but fit frame, a long face with pronounced cheekbones, and narrow, piercing yellow eyes. He has short, slicked-back black hair with four distinctive bangs hanging over his forehead, and Shishio Makoto famously described him as "a man who looks like a wolf." During the Meiji era, he is almost always seen in his blue policeman's uniform and is rarely seen without a cigarette. He is also a married man, though his wife, Tokio, never appears in the main story; Saitō is known to be slightly annoyed when forced to admit to his domestic life, as it conflicts with his cold and terrifying image. He has a noted fondness for simple soba noodles.

As a combatant, Saitō is exceptionally powerful. He is a master swordsman well-versed in the techniques of Shinsengumi-kenjutsu, but his personal style is straightforward and relies heavily on basic attacks. He is ambidextrous, able to switch his dominant hand seamlessly in battle to confuse opponents. His signature technique is the Gatotsu, a refined and immensely powerful thrusting attack. The sheer force behind the Gatotsu is considerable, capable of blasting apart a thick steel door even when Saitō is gravely wounded. Beyond his swordsmanship, he is also highly skilled in hand-to-hand combat, integrating martial arts maneuvers into his fighting style. He possesses immense physical strength, demonstrated when he defeated the much larger Sanosuke Sagara in a fistfight.