Junpei Yoshino is a supporting character who appears early in the story as a 17-year-old high school student from Kanagawa. Physically, he is a thin young man with black eyes and shoulder-length black hair, styled with a side parting that allows a large portion of his bangs to cover the right side of his face. This hairstyle is a deliberate choice to hide a cluster of circular scars on his forehead, which he received from bullies who pressed cigarettes against his skin. He was raised by his single mother, Nagi Yoshino, as his parents are divorced. Before the events of the story, Junpei started a film appreciation club with two friends during his first year of high school, but the club’s classroom was taken over by delinquents, and the bullying he endured eventually drove him to truancy and isolation.

Junpei is characterized as a reserved and apathetic individual who holds very negative views of people and society as a whole. This outlook stems directly from the prolonged trauma of being bullied and rejected by his peers, leading him to expect evil from others as a defensive mechanism. He famously claims that if there existed a button that would kill everyone he hates, he would not press it, but if there was a button that would kill everyone who hated him, he would press it without hesitation. He believes that "indifference" is the highest virtue humans can attain, seeing it as an emotional armor against a world that has consistently hurt him. Despite his misanthropic exterior, his love for his mother is extremely deep and serves as a primary force pulling him back from total darkness. He also has a strong, genuine passion for watching movies, enjoying everything from obscure B-grade films to major franchises, and he prefers watching them in theaters as a form of escape.

The character’s primary motivation shifts from a desire for escape to a thirst for revenge. Initially indifferent, he is easily manipulated by the cursed spirit Mahito, who feeds into Junpei's pain and encourages him to transform his apathy into active hatred and a desire for retribution against those who tormented him. This shortsightedness blinds Junpei to Mahito’s true, monstrous nature. His motivation becomes devastatingly focused after the death of his mother, who is killed by a curse attracted to a special-grade object planted in their home. Mahito manipulates Junpei into believing a classmate is responsible, driving him to seek violent revenge at his school.

In the story, Junpei’s role is that of a tragic foil and a cautionary figure. He serves as a mirror to Yuji Itadori, showing what a lonely and wounded teenager might become under different circumstances. His interactions with Yuji form a key relationship, as the two bond quickly over their shared love of films. Yuji’s open and straightforward nature begins to break through Junpei’s cynical shell, offering a genuine friendship and a path toward a better life at Jujutsu High. His other significant relationship is with Mahito, who acts as a twisted mentor, exploiting Junpei's vulnerability to use him as a tool against Yuji. His relationship with his mother, Nagi, is his most precious emotional anchor, and her death is the catalyst for his final, tragic descent.

Junpei’s character development follows a tragic arc from victim to a potential ally before being cut short. Starting as a powerless target of bullies, he gains cursed abilities through Mahito's manipulation but hesitates to kill, realizing it would taint his soul. After losing his mother, he abandons this restraint and attacks his school, only to be stopped by Yuji, who pleads with him to stop and join him at the technical college. Just as Junpei begins to reconsider his path and see a future beyond revenge, Mahito appears and kills him by forcibly transfigured his body into a distorted creature, using him as a final pawn to torment Yuji.

As for notable abilities, Junpei was naturally able to see curses, demonstrating latent jujutsu potential. However, before meeting Mahito, he was an ordinary, weak civilian unable to defend himself. Mahito altered Junpei’s brain to forcibly awaken his innate technique, Moon Dregs. This technique allows him to generate a potent poison from his own cursed energy and channel it through a summoned shikigami in the form of a large jellyfish, also called Moon Dregs. This shikigami can adjust its size and the potency of its poison at will, using its tentacles to restrain enemies and secrete toxins that paralyze and burn the victim. The shikigami can also wrap around Junpei’s body to act as a defensive shield. Despite lacking formal training, his innate talent was considerable, and he was able to briefly hold his own against Yuji in combat.