He is a former student at Satozakura High School who stopped attending in his second year due to frequent bullying from several classmates. He possesses an indifferent attitude towards other humans from being bullied and the lack of empathy/concern from classmates and teachers when being bullied.
He witnesses three male students, who he recognizes as former bullies of his, killed by Mahito in the theater during a screening of a movie. He runs after Mahito and asks if he was the one who did that and if he can also do the same. The two begin to spend more time together, with Mahito teaching Junpei about curses and other various things relating to jujutsu sorcery.
He quickly becomes friends with Yuuji when the latter asks what movie he was seeing in the theaters when the three students were killed and learns that Yuuji has also seen it before, in addition to many other obscure and niche films.
(Source: Jujutsu Kaisen Wiki)
