Junpei Iori is primarily present as a Phantom Idol, or cognitive teammate. He has not yet made any appearance in the main story.
Junpei’s first appearance in the P3RE Collab is in Chapter 5, where the group reunites with him after he and Fuuka had been separated. Junpei acted as bait to lead Arcana Emperor and Empress away from Fuuka so she could look for the others, and ultimately had to run from them until he found a door that led him to the Iwatodai Strip Mall. He was starving from all the exercise, but luckily after meeting up with everyone, Shun was able to make him some ramen.
From there he joins the group, ending up with the majority group when Riko, Yukari, and Akihiko get separated from everyone else. In Chapter 6, when Junpei actually becomes playable, he cracks jokes and keeps things light as the group progresses through Tartarus, but also helps protect Fuuka and even mentions Chidori during the Arcana Lovers fight.
In Junpei’s Dream Chapters (bonus events the player can unlock), he and Nagisa are transported to various locations around Tokyo and Tatsumi Port Island. His final three Dream Chapters, which require special items to unlock but have a bit more to them, are:
The Doll is Staring (unlocked by reaching the 5th floor of Tartarus: Eternal Night with Junpei on your team): Wonder gets a Japanese doll, and Junpei mentions a ghost story he's heard about one, which reminds Wonder of someone else he knows who likes urban legends and the like. He and Junpei are transported to outside Victory Kitchen, where they run into Kiyoshi Kurotani, who talks with Junpei about being "chosen ones" secretly protecting their schools.
Reading Souvenirs (unlocked by reaching the 10th floor of Tartarus: Eternal Night with Junpei on your team): Wonder gets a book cover, which prompts Junpei to admit he doesn't read much. He and Junpei are transported to the bookstore in Shinjuku, where they run into Tomoko Noge, who's currently working, and she connects with Junpei over their shared interest in baseball before recommending him some baseball-related books.
The Prince's Gift (unlocked by reaching the 15th floor of Tartarus: Eternal Night with Junpei on your team): Wonder gets a bouquet of flowers, which reminds Junpei of Chidori, eventually leading to him admitting he wants to ask her out when she's out of the hospital. He and Junpei are transported to Kichijoji, where they run into Ichigo Shikano, who misinterprets the situation and fears Wonder is giving the bouquet to Junpei. After they clear things up, Chizuko Nagao also comes by, and she and Ichigo both give Junpei suggestions for how he can be Chidori's "prince" and ask her out.
Junpei can also be encountered in the cognitive Paulownia Mall, and later the cognitive Iwatodai Dorm, in his casual outfit, and interacted with there.
Junpei’s Persona Hermes is categorized as a Fire type, and resists Fire while being weak to Wind.
Hermes is a Guardian Persona, meaning he’s good at protecting teammates and keeping them alive. His trait grants Junpei one "Competitive Spirit" at the end of each ally's turn (stacks up to 15):
At 10 Competitive Spirit: All allies' attack stat and fire damage dealt are buffed
At 15 Competitive Spirit: Junpei's damage reduction stat is buffed, and all allies' attack stat and fire damage dealt are further buffed
Junpei also gains 1 charge of "Power Slash" every two turns. Power Slash is activated when he takes damage (not counting damage from a damage over time (DOT) effect), and is considered an additional effect. It gives Junpei 6 Competitive Spirit, deals physical damage (based on Junpei's max HP) to one enemy with a guaranteed critical, then inflicts the target with "Frenzy Ball", and grants all allies shield for three rounds. When an enemy with Frenzy Ball dies, Frenzy Ball transfers to the enemy with the highest HP.
His fire attack skill hits one enemy and, if the target has it, detonates Frenzy Ball, dealing additional fire damage to all enemies, then reduces their defense and buffs all teammates' fire damage (based on Junpei's max HP) for two rounds. His physical attack skill hits one enemy (and if the target has Frenzy Ball, this attack is a guaranteed critical), then reduces their defense and buffs all allies' damage dealt (based on Junpei's max HP) for two rounds. His support skill gives Junpei "Focus of the Battlefield" and taunts all enemies for two rounds, then buffs all allies' critical hit effect based on Junpei's max HP for two rounds (the buff is increased for Fire Assassins and Sweepers).
Focus of the Battlefield: Junpei takes 40% of any non-DOT damage that would be dealt to any allies (also reducing the amount of damage as he takes more hits), and is immune to mental status ailments and knockdown. He also has a high chance of inflicting burn on any attackers who he takes damage from, and automatically unleashes Power Slash on his next turn if he doesn't take any non-DOT damage.
His passive skills buff all allies' attack stats, buff all allies' damage dealt if at least one Fire Assassin or Sweeper is present at the start of battle, heal Junpei for a small percentage of his max HP at the end of his turn, and heal all allies based on Junpei's max HP if any teammate falls below 30% (this healing can trigger once, but is recharged whenever Junpei uses his support skill).
As in P3RE, his melee weapon is a two-handed sword. However, as the members of SEES don’t have ranged weapons, he instead has an ability called an “Aid Skill” that can be used once per battle at any time, and doesn’t use up his turn. Junpei’s Aid Skill grants all allies a shield (based on their own max HP) for 3 rounds. His Theurgy, Hack n' Blast, can be seen in this video from 2:33 - 2:47, and it requires 140 points in his Theurgy Gauge to use; it hits one enemy with a physical attack that ignores type resistances, then reduces their Down Points by 4 (if the target has less than four Down Points, the extra points are deducted from another random enemy). It then buffs all allies' attack and critical hit effect (based on Junpei's max HP) for 4 rounds (stacks up to 2). Junpei gains double Theurgy energy if a Fire Assassin or Sweeper is present at the start of combat.
His recommended card sets are 1) 2 of Wands (Control)+ Knight of Wands (Departure), 2) 4 of Swords (Reconciliation) + Prince of Wands (Creation).
The game recommends teaming him up with 1) the Persona 3 protagonist, 2) Lion Dance Rin and Howler.