Tōshirō Hitsugaya is very young by Soul Society standards. Born in Rukongai, he became the youngest ever to reach captain rank. Thus, he is called a "boy genius" or a "child prodigy" by many. Hitsugaya is very short in height comparable to Rukia Kuchiki's. He also has spiky white hair (which tends to draw attention in the human world) and teal-emerald eyes, which evokes his ability of controlling ice and water. He wears standard captain's clothing, save for a green sash around his shoulders which holds Hyōrinmaru, his zanpakutō.

Hitsugaya is generally mature and serious, in contrast to his free-spirited lieutenant, Rangiku Matsumoto, although he can sometimes be short-tempered and irritable. Despite their personality differences, he and Matsumoto seem to be very close. Hitsugaya is shown to also be easily irked by others goofing off or drawing unneeded attention to themselves, as shown when he led a team of Soul Reapers into Ichigo Kurosaki's school. He hates anything that he deems childish and hates being described as something close to a child, evident in a filler episode when he meets Ichigo's sister, Karin, who pats him over the head further angering him. This is also shown more in the omake chapters, where he is repeatedly mistaken for a child, much to his annoyance. Fitting with the nature of his zanpakutō, Hitsugaya dislikes summer and warm weather in general. He is also rather talented as a soccer player as seen when he singlehandedly helps Karin's team level a 5-4 scoreline in a filler.

Hitsugaya's intuition is extremely sharp, and he is one of few captains to correctly suspect Gin Ichimaru of foul play, though he did not suspect Sōsuke Aizen of the same. During his duel with Ichimaru in the anime, the latter remarks that Hitsugaya is the embodiment of a heavenly guardian that would be reincarnated once in a few centuries. This is attributed to his given name, which is a homonym for a casual Japanese term meaning "amateur", possibly referring to Hitsugaya's young age and inexperience relative to the other captains of the Gotei 13. He is overly protective of Momo Hinamori, due to the fact that they were old childhood friends, promising to kill those who harm her.

A running gag is Hitsugaya getting annoyed when anyone refers to him by anything other than his title, Captain Hitsugaya. Hinamori has called him by "Tōshirō," ("Hitsugaya-kun" in the Japanese manga and anime) and both she and Ukitake have occasionally used "Little Shiro" (Shiro-chan) — a nickname meaning "Whitey" (Snowy in the English manga) in reference to his hair — in both cases without repercussion, however. In the Shinigami Golden Picture Book sketches (omakes found at the end of episodes of Bleach), Jūshirō Ukitake comically gives Hitsugaya candy and other food because "Jūshirō" and "Tōshirō" sound similar and because they both have white hair, making them both "Shiro-chan." The first character in Hitsugaya's given name, Tōshirō, means "winter," which is appropriate given his white hair and ice type zanpakutō. Ichigo also calls Hitsugaya by his given name without the use of honorifics, much to his irritation, making him one of three captains Ichigo refers to in this manner (the others being Byakuya Kuchiki and Kenpachi Zaraki). In contrast, Orihime Inoue calls Hitsugaya "Tōshirō-kun".