Mai Zenin/Synopsis

History
Mai was born in the Zenin Family, as the younger twin sister of Maki. As a child, Mai always relied on Maki since she was afraid of cursed spirits. Later in life when Maki left the clan, Mai began to resent Maki for leaving her.

Fearsome Womb Arc
Mai accompanies principal Yoshinobu, along with Aoi, to the Tokyo College. After arrive at Tokyo, Mai along with Aoi meets up with Megumi and Nobara. As Megumi wonders why they're here, Mai tells them that she came to because she was worried for them since their classmate had just died. Mai then starts to insult Yuji and how Megumi must be relieved that Yuji is dead.

When Aoi starts fighting Megumi, Mai prevents Nobara from assisting Megumi in the fight. Nobara insults Mai about her looks, which angers Mai enough to shoot Nobara several times with rubber bullets. Suddenly, Maki shows up and distracts Mai long enough for Nobara to put Mai in a choke hold. Aoi shows up and says that they are leaving, where Nobara releases Mai. As Mai leaves, she proclaims that things will be different during the Kyoto Goodwill Event.

Kyoto Goodwill Event Arc
On the day of the Kyoto Goodwill Event, Mai arrives at the Tokyo College along with the other Kyoto students and faculty. As the Tokyo and Kyoto students meet, Gojo shows up and hands the Kyoto students souvenirs from his trip. As Gojo reveals that Sukuna's host, Yuji, is alive, Mai is focused on the souvenir. Afterwards the faculty explain how the event will go, Mai attends a pre-event meeting along with the other Kyoto students. As Yoshinobu tells them that they will kill Yuji during the event, Mai talks with the others about how they will handle it.

When the event starts, the Kyoto students manage to locate Yuji and attack him. The Kyoto students are then forced to retreat by Aoi, which Mai splits with Mechamaru. Later, Mai helps out Momo by knocking Nobara out with a rubber bullet. Mai then calls Momo, and tells her to continue supporting them by flying in the sky.

Suddenly Mai is confronted by Maki, Mai plans to fight Maki alone instead of calling for backup. As Mai fires shots at Maki, Maki easily cuts them in half. As Maki manages to close the distance, Mai fires more bullets but Maki easily counters them. After Mai fires all six shots from her revolver, Maki starts to close the distance again before Mai can reload. When Maki gets close enough, Mai uses her cursed technique that allows for her to construct another bullet but at the cost of her cursed energy and fires it at Maki. Maki manages to catch it, and Mai is defeated. Mai suddenly tells Maki that she would have been fine working as a servant to the clan as long as Maki was with her, which Maki responds by saying that she would have regretted it. As Maki leaves, Mai remembers how her sister said that she won't leave her and calls her a liar.

When the site is invaded, Mai notices that a screen is covering the site. Mai is with Nobara when they come across Utahime and an intruder. As Mai, Nobara, and Utahime prepare to face the intruder, all of them notice that the screen has been raised and the intruder leaves. After the intruders have been dealt with, Mai attends a meeting where they all agree to continue the event and play baseball. During the game when Maki hits Aoi with the ball, Mai, alongside with the rest of the Tokyo and Kyoto students, tells Maki that that was a good pitch. After the event is finished, Mai heads back to Kyoto along with the other students and faculty.

Shibuya Incident Arc
Kokichi Muta manipulates events to ensure his classmates would be out of harm's way during the Shibuya Incident. On the train ride to Shibuya, Mai along with the other students and Utahime gives Miwa privacy as she cries over Mechamaru's death and her own powerlessness. Mai asks Utahime about what Mechamaru did, and the woman replies that it doesn't matter anymore as he's already dead.

At 11:36pm, Mai arrives with her allies in Shibuya and opens fire on Geto in one last attempt to rescue Satoru Gojo. Mai uses a sniper rifle in an attempt to shoot Pseudo-Geto but fails to. The curse user however, takes note of her boldness.

Itadori's Extermination Arc
Mai is mentioned by Naoya Zenin, who wonders if she's dead.

Perfect Preparation Arc
Mai is inside of the Zenin Household's weapon warehouse heavily injured presumably from fighting her father when Maki walks in. Maki is surprised to see Mai who weakly asks her why'd she came and calls her an idiot. She is left unable to do anything as she watches Ogi overpower Maki in their sword dual. After Maki is defeated, Mai along with Maki is dragged by Ogi as he talks about how his children must not hold their parents back.

Mai asks her father if he knows about how holding each other back is a virtue in this country. Ogi doesn't respond and instead throws the two down a flight of stairs. He then tells Mai and Maki that the room they're currently in is used for training and punishment. He adds on that there are multiple grade 2 and low leveled cursed spirits in the room and that the cursed spirits are staying back in fear of him but will soon come out to eat them. When Ogi leaves, Mai manages to stand up and moves to Maki where she senses a heartbeat. Mai comments on how tough Maki is before adding on how she suspected this situation would happen someday. She then positions Maki's head on her lap and leans over her, bringing their faces together. Maki then awakens to her and Mai sitting at an unknown location metaphorically presented as a beach. Mai begins speaking of her cursed technique and says that her injuries are too heavy for her to survive what she is planning to do. Mai then gets up and begins walking into the ocean and says goodbye to Maki. At first Maki doesn't understand what Mai is doing, so she calls out for her sister. Mai explains that twins are born with their cursed potentials tied to each other and so as long as Mai, who is not planning on becoming stronger, is alive and stagnant, Maki will remain where she is too. Mai admits to Maki, that for her to fully utilize her heavenly restriction, Mai will have to die. She takes away all of their cursed energy and gives Maki a parting gift — a reed branch, signifying a promise for Maki to destroy everything, namely the Zenin clan. When Maki comes to her senses back in the training room, she tries to wake Mai up, but her sister is dead. In her hands, instead of what used to be a reed branch, Maki finds a new weapon. After propping Mai against the wall, exorcising all the curses in the room and killing Ogi, Maki addresses her dead sister, saying that she's getting started on fulfilling the promise.

When Maki is finished massacring the Zenin clan, Mai's body is handed over to Momo.