Yukine is a second year student at Tomoya's school. She is mild-mannered, polite, and compassionate. Yukine is found at the reference room during lunch breaks serving coffee and snacks to visitors. She exudes an aura of comfort and wisdom.
Yukine is fond of performing spells from a book; which consists of performing a gesture, followed by repeating a phrase 3 times. She will then reveal the effect of the spell after that, which usually turns out to be something strange and not desired.
Yukine's family was very strict. Her brother, Kazuto, was a free spirit and felt suffocated living in that atmosphere. He started becoming rebellious and tries to stay away from his home for as long as possible. He also made friends among fellow free-spirited rebels. They were labelled as delinquents but Yukine still loved her brother.
In Yukine's first year, Kazuto met with an accident. Yukine met his friends at his funeral and became their friend. She recognized them as being similar to her brother. Yukine wasn't able to mix with her peers, but instead found friends among delinquents. Kazuto's friends would occasionally visit her during lunch and she uses the reference room as their meeting spot.
Yukine was first seen when Tomoya chose to take a nap at school on April 18. Yukine welcomed him and made him coffee. She explained that the room used to be the library before it moved to the new school building and it turned into a reference room which kept old books. Tomoya then started to visit the reference room daily.
4 days later, Tomoya revealed the reference room and Yukine to Youhei. Youhei fell in love with her and made several attempts to confess his feelings for her, which caused him to be doused in boiling water. Eventually, she got used to their daily visits and prepared coffee for them in anticipation.
One day she announced that a friend will be visiting her. A badly injured man crawled through the window and she gave him coffee. Youhei later revealed that Yukine is a good student but she had been in contact with men like that for a while. Later, Tomoya confronted Yukine about the man and she explained that she enjoys helping people. Tomoya realized that Yukine had no one to turn to in her time of need and decided to be there for her.
Yukine acquired a cooking stove and started preparing meals. One day, a boy named Yuu confronted Yukine claiming that one of her friends had kidnapped his sister. Youhei taught Yuu to be independent and to search for his sister. Yuu was inspired by Youhei's speech and became attached to him. Later, Tomoya, Yukine, Youhei, and Yuu ran into Yuu's mother after school. She was worried about Yuu's absence from home and school. Yukine explained that Yuu had learned to become independent and sent Yuu back.
One day, Yukine felt tired and requested to take a nap on Tomoya's lap. In her sleep, she was calling for her brother. Later, one of Yukine's friends revealed that Yukine missed her brother and that Tomoya closely resembled her brother. Tomoya wondered if Yukine only saw him as a brother substitute.
Yukine invited Tomoya to pay respects at her brother's grave on the anniversary of his passing. She explained her history and Tomoya gave her a kiss.
Later, Yuu's mother allowed him freedom and he visited Tomoya, Yukine, and Youhei. He invited them to his school's festival.
Yukine proved to be a skilled basketball player.