The shy younger sister of Tenma Tsukamoto, Yakumo was born with a variety of gifts. Not only is she able to read the thoughts of those who like her (as well as her sister and pet cat Iori), but further in the story it is shown that she also has the ability to communicate with ghosts.
Skilled, reliable, smart, and beautiful, Yakumo is one of the most admired girls in the school. Despite that fact, she has never had a boyfriend. This is due mostly to her mind-reading ability, as she can easily read the secret intentions of any boy who tries to approach her. The only exception is Harima, whose heart belongs completely to another girl; thus, his mind is completely opaque to her. Even though Yakumo is the younger sister of the Tsukamoto family, she handles most of the household chores. She is also a member of the Tea Club.
Yakumo eventually grows a strong friendship with Harima; which leads him to trust her with the critiquing of his manga drafts and keeping his pastime a secret. Eventually she takes on the task of assisting him on the actual inking of pages, which causes them to spend many hours together alone. Yakumo eventually develops deeper feelings for Harima than just the master-apprentice relationship. She keeps them hidden from him however, as she realizes he is in love with her sister.