No Onyu

Apparently having come from the future, he goes looking for Yuna, who is rumored to bring trouble to anyone who gets close to her. However, despite knowing this, No Onyu confesses to her out loud in front of several students to purposely provoke the Yuna Band.

Suffering from amnesia, he doesn't remember anything about himself: name, personality, history. However, he has a feeling that draws him to Yuna and a notebook that he likely brought with him from the future cataloging the events that will happen to her. He tells her that she will die one year from now and says he'll be with her no matter what. He does slowly regain his memories day by day once future events that haven't been changed pass.

He clearly has an obsession with Yuna, bordering on yandere-like tendencies, in his acts to protect her.

After Yuna was rejected by Shung, she comes across Onyu later at night on school grounds. He is covered in blood from his fight with the twins. Yuna hugs him and he says he's relieved that she is no longer angry. Yuna admits to her selfish thoughts whenever Onyu wanted to talk to her. When she asks him about his story, Onyu happily agrees. He quickly finds himself crying and breaks down, falling at her feet and begging that she not leave him. She promises she won't.

No Onyu turns out to be the son of Yuna and Shung. It is never confirmed if No Onyu is his real name, but it is possibly the name given to him by his parents. There are 3 versions of Onyu that coexist, each having been part of different timelines. Ji-Hyun Park tells Onyu, that he has seen Yuna's death twice, once as a child and another as a young adult. First, there is Bustard, who witnessed Yuna's death as a child. He is able to timeslip and go back to when Yuna was at a young age. This is when Yuna began to recognize the presence of a stalker around her.

After losing his memories at the hospital, he and Yuna eventually leave but are confronted by Crow and Bustard. As Onyu fights Rin, at one point his left eye is slashed with something sharp. Bustard corners Yuna and insists she is only at her happiest with him. She is rescued by her father-figure and given a chance to escape. He pins Bustard to the ground while holding a knife, and his sunglasses slide down his nose, revealing an eye color very similar to that of Onyu and Bustard.

It seems that, somehow, the version of Onyu that was attacked and scarred by Crow is the version of Onyu that went back to when Yuna was a kid and took her in after her parents were killed. He did not attempt to change the past, and was content keeping her safe and happy. This turned out to be the way to keep Yuna safe, by killing Bustard and undoing everything he had interfered with.