Initially, Kasumi was a very anti-social person as she stays away from people in general. As a child, she often lost when playing games with her sister and had to carry out tasks as punishment. This led her to have a very strong aversion to all games. It wasn't until her encounter with Olivia and Hanako Honda that she began to change her outlook on games and people. She initially sought out Hanako and Olivia and created the Pastimers Club to learn English from Olivia, but as time passed she began to appreciate the companionship provided by the two so much that when Olivia revealed the truth that she couldn't speak English, Kasumi forgave her immediately because they were 'already in a club.' She treasures her friendship with Hanako and Olivia dearly, even if it does often lead her to disastrous events.

Kasumi is often seen as a shy and introverted girl. She often speaks in a timid and friendly manner and is often unwilling to impose on her friends. However, her shy demeanor deceptively hides her 'dark side'. When driven to be serious or angered, Kasumi displays another side of her that is cold, sadistic, and sometimes violent. This side of her can intimidate and strike fear into the people around her, even her own friends.