Beautiful and smart, Yakumo is the most admired girl in the school. Despite her many qualities, she has never had a boyfriend, having turned down at least eight would-be suitors.
Because of her many talents, Yakumo is scouted by several school clubs. She declines joining most of them, however, because she dreads having to spend time together with the infatuated Haruki Hanai, who follows her with the intention of joining the same clubs. Thanks to a ban on Hanai decreed by Akira Takano, Yakumo is now a member of the Tea Club along with her best friend Sara Adiemus.
Yakumo seldom socializes with others (especially males) but easily interacts with children and animals, except for dogs. Like Harima, she is very fond of animals. She initially becomes friends with Harima because of his caring nature towards her cat. Her interest in him is further increased when she learns of his ability to communicate with animals.
In the original Japanese dub, her voice actress Mamiko Noto voices her as a very soft-spoken person, and this contrasts with her hyperactive elder sister, Tenma Tsukamoto. This is relatively constant with their character differences.
According to the last Japanese poll performed by the publishers of the manga, Yakumo is currently the most popular character in the series.
Yakumo has an innate extrasensory ability to literally read minds who express interest in her. The only male whose mind she cannot read is Kenji Harima, whose heart belongs to her sister; the only time Yakumo was able to briefly see his thoughts was when Harima mistook her for Tenma. Yakumo can also read her sister's mind and that of her pet cat Iori.
Aside from mind-reading, Yakumo is also shown to be able to communicate with ghosts. She also has reflexive fighting skills that she puts to use when defending herself even when asleep, or when Tenma is insulted. Unlike Tenma, she is well-versed in homemaking skills such as cooking and sewing. If there is a fault within her, it is her tendency to fall asleep practically anywhere, especially when in school. But people (even girls) who have seen her sleeping on a bench near the school compounds have commented that she looks like part of a beautiful drawing of a goddess sleeping.
Yakumo lives alone with Tenma in a relatively large (by Japanese standards) house. No mention is made of the fate or whereabouts of the girls' parents, but it is hinted that they are orphaned. Even though Yakumo is the younger sister, she handles most of the household chores. She does not appear to mind this arrangement, however, because of her love for her sister. Her relationship with her sister also takes an important stage in the plot. Yakumo strongly supports her sister's affection for Karasuma, but when left at home for the whole week while Tenma goes for her Kyoto trip, Yakumo begins to feel alone, and the sight of her sister with Karasuma makes her feel somewhat lonely and unable to express her loneliness even when Tenma asks if she felt lonely when she was away. She also admires her father a lot, as seen in flashbacks of her childhood. It is possible that her affection for Harima is partly due to his resemblance to her father -- at least from behind.
Yakumo's growing friendship with Harima leads him to trust her with the critiquing of his manga drafts and keeping his pastime a secret. Later on, 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. However, she keeps them hidden from him as she realizes he is in love with her sister. She initially decided to give in and support his relationship with her sister, as she regards Tenma as the most important person for her in the world and wants to make her happy. However, once her feelings for Harima became too strong for her to contain, she anxiously declared her affection for him, only for Harima to misinterpret it as her love for manga.
She is also able to communicate with ease with Shuuji (Harima's younger brother), and is even willing to proactively express care for him (such as hugging him to sleep) when she manages to catch his innocent thoughts of admiration for her, as compared to the more perverted thoughts from her male high school peers and other males.
Her classmate, Sara Adiemus, became Yakumo's best friend after she defended Yakumo from a stray dog and is very sympathetic with the trials that Yakumo has to go through when dealing with Hanai. By association with Tenma, she also knows Tenma's close friends Eri Sawachika, Mikoto Suo, and Akira Takano, and has accompanied them and her sister on trips a few times. Her relationship with Eri became strained however, because of her perceived involvement with Harima, whom Eri herself became interested in. That same relationship gets worse when she was believed to have accidentally kissed Harima during the marathon, considering that she slapped Eri previously in the arc, where Eri found out Harima's true love interest and began to ignore Tenma.
After her sister left to the United States, Yakumo took some time to cope with her new lonely environment, until Sara and Harima move to her house. She continued living with Sara after Harima's sudden disappearance, and during the reunion occurred several years later, she still demonstrate to have some feelings for him.
Harima calls Yakumo (little sister) in reference to her relationship to Tenma.