Hijikata Toushirou serves as the vice-commander of the Shinsengumi, a special police force in Edo, where he is famously known as the Demonic Vice-Commander for his strict and fearsome demeanor. His background is marked by hardship; he was born as the illegitimate son of a mistress and a wealthy farmer. After his father died and he lost his mother, he was taken in by his older half-brother, Tamegoro, who became a father figure to him. When Hijikata was eleven, bandits attacked his home, and Tamegoro lost his eyesight while protecting him. In a blind rage, Hijikata retaliated violently and seriously injured the bandits, an act that frightened his other siblings and branded him as an outsider. This traumatic event earned him the childhood nickname Thorny Toshi and set him on a path as a wandering, troublemaking ronin who sought out fights at various dojos.

His life changed when he was taken in by Kondou Isao, the future commander of the Shinsengumi. Hijikata developed an intense and unwavering loyalty to Kondou, viewing him as the soul of the organization and consistently placing his commander's well-being and honor above all else. He also met Okita Sougo and his sister, Mitsuba, at Kondou's dojo. While he developed romantic feelings for Mitsuba, he rejected her confession before leaving for Edo, believing his violent life as a samurai could never make her happy—a decision that created a lasting, bitter rivalry with Sougo, who frequently attempts to kill him, though a grudging mutual respect exists between the two.

On the surface, Hijikata is short-tempered, humorless, and brutal, often threatening his subordinates with seppuku for the slightest infraction. He is the enforcer of the Shinsengumi's forty-five strict regulations, which he himself formulated. Despite this frightening facade, he possesses a deeply compassionate and emotional side that he rarely shows. He often disguises his good or kind actions with exaggerated annoyance. He is known for two extreme personal obsessions: he is a chain smoker who will go to absurd lengths for a cigarette, and he drowns nearly all his food, including desserts and coffee, in a mountain of mayonnaise, a quirk that cost him his popularity with women.

In combat, Hijikata is exceptionally skilled with a sword, demonstrating the ability to defeat numerous enemies single-handedly and even slice airborne bullets. While his abilities are immense and he can best Sougo in a serious fight, his swordsmanship is generally portrayed as just below that of his primary rival, Sakata Gintoki. Despite their constant bickering and mutual disdain, Hijikata and Gintoki share strikingly similar personalities, fears (such as ghosts and dentists), and can fight together with incredible synchronization. Hijikata sees Gintoki as a rival in almost every aspect of life, from battles to drinking contests and games of rock-paper-scissors.

Throughout the story, particularly in arcs adapted for the films, Hijikata experiences notable developments. At one point, he is cursed by a sword named Muramasa, which forces out a weak and cowardly otaku alter-ego named Tosshi, who obsesses over anime and manga. This split personality ultimately sacrifices himself to protect others. Despite his cold exterior, his driving motivation is rooted in regret for being unable to save his brother and a fierce desire to protect the bonds he now has with the Shinsengumi. He secretly writes monthly letters to Tamegoro, demonstrating a loyalty to his past that he rarely admits to in the present.