Junpei first appears in Shin Megami Tensei: Persona 3, as he becomes the main character's best friend throughout the events that unfold during the game. A class clown in school, his character is always associated with comic relief in the game, as he is also somewhat of a pervert and usually gets slapped by Yukari. He constantly plays video games and doesn't pay attention to his schoolwork at all. His Persona mirror his own abilities as they are based mostly of physical attacks. They consist of Hermes and Trismegistus as his initial and ultimate personae respectively, who are from the Magician Arcana.
Junpei is voiced by Kousuke Toriumi in the JP version and Vig Mignogna in the ENG version.
Found crying in a convenience store during the Dark Hour, Junpei was extended an invitation from Akihiko to join the SEES group, which he immediately accepted. Junpei never really had a reason for fighting when he first joins SEES. He thought defeating Shadows during the Dark Hour is just like some kind of game. He really wanted his friends to look up to him as the true leader, instead of Minato, but he later began to realize that he just isn't cut out for it. He becomes jealous at the main character (Minato) because of his unique persona ability, the Wild Card. There were times where he questions his own purpose for fighting, and he found it when he met Chidori. a member of Strega. They first meet when Junpei was observing her sketching by herself.
At first she views him as weird and annoying, but because of Junpei's persistence to get to know her, they start talking. But Junpei is completely unaware that she is a member of Strega. When Chidori ends up at the hospital after a full moon incident she still hated the rest of the SEES members, except Chidori. Junpei tries to talk to her. At first, it does not go well, but his recent visits bring about a personality change within Chidori. She tells Junpei that she did not fear or care for anything until they both met. Eventually, they fall in love with each other as the story progresses. This also leads to Chidori questioning where her loyalties lie; to Takaya, or Junpei. On a full moon, Chidori fights the SEES members and they defeat her. When Junpei tries to stop her, Strega shows up, and Takaya shoots Junpei. As Junpei is about to die, Chidori suprises everyone by bringing him back to life, as it it her persona's special ability. Before she dies, she tells Junpei that she doesn't fear death anymore because she can live inside him, and be together forever (this explains Trismegistus' special ability, Spring of Life). This tragic event triggers a huge change in Junpei's character. After Chidori's tragic death, he loses his will to keep fighting. He questions his own purpose of fighting Shadows; why he wants to fight, and why he does not have a very good reason to do so. But seeing Chidori's sketchbook, which contained detailed and well-done portraits of Junpei, he gains a great reason to keep fighting.
Junpei also makes fast friends with Ryoji Mochizuki when he transfers to Gekkoukan High School later in the game. But on an event on the Moonlight Bridge, it is revealed that Ryoji is the same being as Death. Only this time, Ryoji gained human qualities while living inside the main character, as he was sealed off by Aigis ten years when they first engaged in battle. Junpei becomes frustrated that Ryoji's real purpose is to summon Nyx and bring about the Fall to the world. But Junpei later changes and decides to fight even though it would kill him. The SEES members are successful in defeating Nyx, and move on with their normal lives after peace is restored.
In Persona 3 FES, there is an opportunity that Chidori can be revived after her death. If the player says the right, encouraging words to Junpei, it will trigger a special event leading to her revival. Although upon her recovery, her persona is no longer functioning, and she does not have any memories of anything that has happened. This includes the times Junpei and her spent together. But she wants to meet the man who is always in her dreams, and is determined to see him. It is not stated in FES if Junpei's and Chidori's story continue (if the requirements for her revival are met), but it is up to the player to determine that.
If you choose the Female Main Character in Persona 3 Portable, you can hang out with Junpei for the Magician arcana. Unfortunately, Junpei is not available to date at Christmas Eve or Summer; his Social Link is friendship only. Upon leveling this Social Link, it is also revealed that Junpei's father is an alcoholic (as well as in the Abyss of Time in Persona 3: FES.)
Junpei's personality undergoes many changes. Instead of fighting just for the thrill of it, he decided to fight for Chidori instead. This decision brings about his downfall, and his uprising.
His personality changes from a laid-back person, to that of a determined young man, maturing as the story progresses. Junpei is the best character many people (especially teenagers) can relate to, because he was the only one who seemed like he didn't have a dark secret or a horrible past. He started as an insecure class clown with low self-esteem. But as the story progresses, he becomes more mature, and finds a purpose to why he fights for those close to him. Chidori's death hits him hard, and he resolves to fight to always protect her. This awakens him to his new persona, Trismegistus.
Junpei's attacks are mainly physical based. He can also cast some relatively weak fire magic making him an advantage as a physical attacker when you want him in your party. He can easily be knocked down by any Wind-type (Garu) attacks though, as it is his main weakness.
In Persona 3 FES, Junpei fights alongside SEES once again as a new threat emerges from under their dormitory. As they traverse deeper into the Abyss of Time, the mystery behind it and also the reason why they're stuck within the dorm gets clearer. He is not bothered with the thought of fighting shadows again, unlike Yukari. When the player accesses one of the doors, Junpei's past is revealed. It was the day when he awakens his persona for the first time (not to mention Akihiko finding him crying at the convenience store). It is revealed that his father is an alcoholic and Junpei does not want to become a failure like him. He also meets Koromaru for the first time.