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.
Junpei's mother, Nagi, invites Yuuji to dinner at their house and, afterwards, Yuuji and Junpei watch a movie together. After Yuuji leaves, Junpei covers his mother with a blanket, who has fallen asleep at the dinner table after too many drinks during dinner, and retreats to his room for the night. His mother awakens to find one of Sukuna's fingers on the table and is killed by a curse that was attracted to the finger.
Mahito manipulates Junpei into believing that Shouta, the bully who was responsible for the cigarette burns hidden by his hair, had cursed him and his mother, when it was he who planted the finger. Junpei shows up at the school in black pants and one of his mother's black jackets in mourning. He puts all the students in an assembly to sleep and approaches Shouta to ask if he was the one who left that at his home. He starts to beat Shouta and plans to kill him when Yuuji shows up and asks him what he's doing.
After Junpei explains to Yuuji what happened to his mother, Yuuji urges Junpei to join him at Tokyo Metropolitan Curse Technical College and explains that there are a lot of great senpais and teachers he can rely on, reassuring him that together they would find whoever cursed his mom and punish them with their own hands.
At this moment, Mahito appears and immediately restrains Yuuji, who begs for him to run away. Junpei attempts to reassure Yuuji that Mahito isn't a bad person, but recalls the experiments he did on humans and realizes the mistake he's made. Mahito tells Junpei he's a fool and uses his Idle Transfiguration technique on him and has him attack Yuuji. Yuuji takes the hits and begs Sukuna to save Junpei and heal him, but is promptly refused.
Junpei dies not too long afterwards due to the transformation.
He learned how to use jujutsu and summon shikigami from Mahito. His Shikigami is Moon Dregs (orizuki), a giant jelly-fish that paralyzes and poisons its victims.
