Yuma Tonami attends the same high school as protagonist Takaaki Kouno. She sports neck-length aqua-blue hair, green eyes, and average height. Her personality fits the tsundere mold, frequently challenging Takaaki while cloaking her underlying romantic feelings behind a self-proclaimed rivalry; Takaaki himself remains largely indifferent to her provocations.

Her interactions with Takaaki often involve confrontation or competition. One notable incident involves a bicycle collision sparking her self-declared rivalry. Despite her insistence on this dynamic, she blushes and stammers when questioned about her motivations, betraying her unacknowledged affection. Her closest friend is class representative Manaka Komaki.

Yuma possesses family ties to the Kurusugawa Electronics conglomerate through her grandfather, positioning her as a potential heir and classifying her as an ojou (young lady of status), though this aspect rarely surfaces in her daily school life.

Throughout the *To Heart 2 adnext* OVAs, Yuma participates in group activities with Takaaki and other female characters. In the first *Another Days* OVA, she joins others attempting to rewrite Takaaki’s memory after amnesia, comically asserting her own version of their relationship. Later, in the "First-Name Basis" OVA episode, she actively pursues Takaaki—alongside other girls—demanding he address her without formal honorifics. This episode reveals she was already on a first-name basis with him, unaware she had achieved this status earlier, and references her grandfather's corporate role.

Karin Sasamori later nicknames her "Yuma-chin," much to Yuma's annoyance, especially when Karin probes about personal matters. Her tsundere traits persist, consistently masking genuine feelings behind rivalry or irritation.