She has green eyes and wavy dark hair and wears the same uniform as her friend Mato. Her beauty is comparable to an adult's. Mato often comments on how tall Yomi is; but Yomi says she has a "complex" about being tall. She is presumed to be 160cm tall, very close to Black Rock Shooter's height.
Yomi is a gentle, good-natured girl who speaks politely and calmly; she is very mature for her age. She is supposedly a member of a well-to-do family, staying for some time in Germany before moving to Mato's neighborhood. She also revealed that her frequent moving from one place to another has caused her to develop a sort of distant attitude towards others.
However, she is prone to having very dark feelings of loneliness and jealousy; she tends to keep these to herself. Yomi has an "other self" which is known as Dead Master.
She seems to enjoy volleyball, which she picks up out of a desire to be with Mato on the court.
She is first seen by Kuroi Mato on the first day of school walking out of a car and continuing on alone. After the school's welcome ceremony, Mato catches up to her eventually and introduces herself, and while Yomi is initially startled by Mato's up-frontness, she warms up to her quickly. She introduces herself as simply "Takanashi". She and Mato go on to become inseparable, although she is much more passive in her speech and thinking than Mato.
Mato's skill on the basketball team encourages her to join the volleyball team, and they start to hang out together. Mato eventually shows Yomi a place near her neighborhood where there is a large view, and buys her a phone charm identical to her own.
However, when Mato and Yomi are put in separate classes, they start to drift apart. When playing volleyball one day, Yomi sees Mato get injured and be tended to by Yuu. Later, Mato tries to introduce Yomi to Yuu and have the three of them hang out together, but Mato ends up spending more time with Yuu.
Yomi starts to feel left out, and ends up spending more time watching Mato and Yuu hang out far away from her. Internally, she begins to feel jealous, allowing her "other self", Dead Master, to take over her and possess her. This causes Yomi to vanish from Mato's world.
Black★Rock Shooter, fused with Mato, fights against Dead Master, with both being seemingly equally matched. Dead Master traps Black★Rock Shooter in a web of chains, and it seems the Dead Master will win. However, Black★Rock Shooter gains the strength to break the chains with her bare hands and confront the Dead Master. Black★Rock Shooter embraces her in that instant, and the Dead Master attempts to resist; unable to handle it, she flies out of Yomi's body and disappears, leaving an unconscious/sleeping Yomi in Black★Rock Shooter's arms.
The post-credits reveal that Yomi and Mato have reconciled, although while both invite Yuu to come with them, they are still oblivious to Yuu's dark state...
- A potential kanji writing of Yomi (ヨミ), 黄泉, means "underworld".
- Her surname, "Takanashi", is written with the kanji "小鳥遊" for "little bird play". Mato, not knowing how to read Yomi's written surname correctly, refers to her as "kotori-asobi" (小鳥遊), which literally means "little birds play". Yomi later explains to her that her last name is read as "Takanashi" (鷹無し), which means "no hawks", and tells her that her surname stems from the saying that "When the hawks are away, little birds play".
- The surname "Takanashi" could also be spelled with the kanji for "takai" (高) for "tall" and nashi (無) for "not", i.e. (高無) "not tall". It can be seen as a pun since Mato, right before Yomi's name is explained, mentions that she thinks Yomi is, in fact, tall.