Tatsuki Arisawa

is a student at the Karakura High School, in the same classroom as Ichigo Kurosaki, her childhood friend and karate partner. When they were children, Tatsuki regularly saved Ichigo when he was getting beaten up, as well as cheering him up when he was upset. A tomboyish karate practitioner with dreams of becoming a vale tudo champion, and already ranking Japan's second strongest in her age group, Tatsuki seems to possess no stereotypically feminine qualities as is typical of female manga characters. Her only obvious feminine trait is how she writes her name: using hiragana: (たつき), though this is because she thinks it looks feminine while she considers the kanji of her name un-cute.

Tatsuki is best friends with Orihime Inoue and is constantly protecting her from men looking for a date, to say nothing of Chizuru Honshō, the school lesbian, who often approaches and grabs Orihime's breasts. Tatsuki has done this since middle school, when she beat up some bullies that were trying to cut Orihime's hair, and had already done so once before her brother died. After that, Tatsuki promised to protect her from anyone who dared to make her cry, and she and Orihime became best friends.

In the earlier parts of the series, Tatsuki mainly hangs out with Orihime, and also protects her from the hollow Numb Chandelier, but gets injured. When Orihime's powers awaken, Tatsuki in turn gets protected by Orihime. Soon after that, Tatsuki begins to see hollows clearly, but never mentions it to Orihime. Tatsuki made Orihime promise that she will come back alive before Orihime goes to Soul Society. Later on, during the first invasion of the Arrancar, Tatsuki nearly dies at the hand of Yammy, who uses his special technique called Gonzui to suck up human souls. A timely rescue by Orihime and Chad saves her. Tatsuki also witnesses the battles between Ichigo, Tōshirō Hitsugaya's team and the arrancar. When Orihime gets taken away to Hueco Mundo, Tatsuki is noticeably upset at her sudden disappearance, punching Ichigo's head through a window and shouting at him for not telling her what he knows. She is later accompanied by Keigo Asano and Mizuiro Kojima to Kisuke Urahara's Shop and sees Ichigo leaving for Hueco Mundo.

Like many of Ichigo's friends, Tatsuki is spiritually aware to the point that she can clearly see ghosts, hollows, shinigami and other such supernatural phenomena, though she lacks any unique spiritual powers of her own. She is strong enough to survive spiritual attacks most humans are incapable of dealing with, though this may be due to her karate training. She also has the ability to sense Orihime from almost anywhere, even across dimensions; she is able to sense Orihime when she travels to Soul Society, but remarks that she can't sense her presence after Orihime is taken to Hueco Mundo.