Kaina Tsutsumi/Synopsis

History
When she was a young girl, the President of the Hero Public Safety Commission invited her to join the agency. With a bright sense of optimism at becoming a great Pro Hero, she accepted. For years she was trained conscientiously until she became really skillful with her Rifle Quirk, becoming not only a popular hero but also the best marksman in all of Japan. Her skill and precision with her shots was admired and envied by the likes of Snipe and other heroes with long-range Quirks.

However, the Commission primarily used her for classified missions, secretly disposing of Villains planning to commit acts of terrorism and corrupt Heroes seeking more fame and fortune. At first, she carried out these orders efficiently and without question, but over time the stress of the seemingly never-ending string of murders began to get to her, all the while needing to keep the front as a public Pro Hero. One day, she came across a group of kids who wanted to shake her hand, leading her to hallucinate her completely bloody hand, and resulting in her questioning her purpose as merely just a cog to maintain the phony illusion of their society.

Kaina Tsutsumi's disillusionment came to a breaking point when the then-President of the Public Safety Commission ordered her to make two corrupt Heroes who were taking advantage of the system "go missing". She expressed her reservations about the order, questioning whether the lack of transparency was akin to brainwashing. When his insistence on the act's necessity failed to convince her, the President gave Kaina Tsutsumi a veiled threat. This proved to be the last straw, with Kaina Tsutsumi killing him in response. She was promptly arrested and incarcerated in Tartarus: the Commission covered up the nature of her arrest, claiming that she killed a fellow Hero in an argument instead.

Paranormal Liberation War Arc
Within hours after the end of the Paranormal Liberation War, Tomura Shigaraki, possessed by All For One, started an assault on Tartarus with his accompanying Nomu. Inside the prison, an EMP attack shuts down Tartarus' monitoring system, and ends up freeing all of its inmates, including Kaina Tsutsumi. As the inmates start attacking and killing the guards, Kaina Tsutsumi makes her escape, noting how the system isn't responding to their Quirks, before noticing an inmate banging on his door, unable to escape. Kaina Tsutsumi uses her Quirk to open the door, as Kai Chisaki comes out running, desperate to reach out to his boss.

Shortly after the breakout, Kaina Tsutsumi is approached by All For One, who is honored to meet the "treacherous hero", while she muses about targeting him sometime in the past. All For One asks Kaina Tsutsumi for a favor, considering her to be leagues above the other escapees. He explains that a boy will be leaving U.A. High School soon, so he wants her to capture and bring the boy to him. Kaina Tsutsumi interrupts, insisting that she has no interest in anything but escaping Tartarus. All For One convinces her that so long as the boy lives, the collapse of hero society she sought will never arrive, so she reluctantly accepts his proposal.

All For One notices the mentally broken Kai muttering about his boss, and asks if he's a friend of hers. She denies this, but sees use in Kai's broken state, so she brings him with her. To seal their deal, All For One offers a gift, and transfers Air Walk to her.

As she prepared to head out, Kai reveals to her that he knows their target, as he ruined his plans in the past, saying he has a severe case of "hero syndrome". Kaina Tsutsumi is surprised by this revelation, seeing this as fate, and asking him to come along and identify him for her. He agrees on one condition, and while she initially insists on not taking up any old grudges of his, he quickly denies this, saying he just wants to see his old Boss again.

Tartarus Escapees Arc
On a rainy night on a rooftop, Kaina Tsutsumi, accompanied by Kai, fires a bullet and hits Izuku Midoriya's phone using a sniper rifle protruding from her right elbow. With the phone pinned to a nearby wall, Kaina Tsutsumi talks to Izuku through a microphone inside the bullet. She tells him that if he complies, she will keep his limbs intact. Izuku then moves, prompting Kaina Tsutsumi to fire at him. As he is avoiding her shots using Blackwhip, she curves a bullet and lands a hit on Izuku, although he figures out her location and makes his way towards her.

While retracting her Rifle, Kaina Tsutsumi notes that this is the first time anyone has deflected two of her bullets. Kai then blankly mutters that he wants to get back to his Boss. Annoyed, Kaina Tsutsumi picks him up and tells him to hide, and begins having second thoughts about bringing him along. Using Air Walk, she goes to finish her mission.

Kaina Tsutsumi swiftly moves through the air and shoots two bullets at Izuku from different angles, with one getting deflected and the other grazing Izuku's stomach. As she shot once more, Izuku unleashes a massive smokescreen, catching her off guard. Now knowing that Izuku has multiple Quirks, she repositions up away from the smoke, aiming at Izuku again. She shoots at moving figures, only to learn that they are decoys. She tells Izuku that he can't hide in the smoke for long, but is surprised when he charges out of a building and grabs her arm.

As Izuku asks where All For One is, she proceeds to punch him with the end of her Rifle, as he falls through the sky. He tries to ask why she's working with him if she used to be a Hero, but she tells him he couldn't understand as all he's ever seen is the manufactured sense of order, and not the real truth. Continuing to shoot at him, she explains about her history working for the Hero Public Safety Commission, the dirty work she did for them, and how this ended with her killing the former President. Izuku is shocked by the revelation, believing the news that she killed a Hero in an argument. Kaina Tsutsumi tells Izuku that he's ignorant, that a superpower society is an illusion, that bringing back the status quo will just have history repeat itself, and that no matter how bad a world ruled by All For One would be, it'd be much more transparent.

However, to her surprise, Izuku admits that he was ignorant, and that he's begun to understand that most things in the world aren't black-and-white, but shades of gray, and that is why he wants to be someone to extend his hand to help those in need. Kaina Tsutsumi, unconvinced by Izuku's resolve, seeing it as more of just phony education, decides that she needs to distract his focus, pointing her Rifle directly at Kai, who's on top of a nearby building.

Amplifying her muscles as much as possible, she taunts Izuku, telling him that their Hero education has made it so that anybody dying on their watch, no matter who it is, will weigh in on their conscience, shooting the bullet at Kai with the intention of using Izuku's hesitation to catch him off guard and finally stop him. However, he once again sends her into complete disbelief as he manages to blast his way forward faster than the bullet, realizing he had already changed directions the moment she was about to shoot.

She attempts to realign herself to try again, but he suddenly appears blasting her Rifle arm with a Faux 100 Percent: Manchester Smash, breaking it and taking her out of the fight. As her Air Walk starts to give in and she begins to fall, she thinks about how Izuku didn't hesitate at all to save Kai, even though he recognized him as an enemy, like it was the most natural thing, and wondered when it was that the Commission's "platitudes" started to make her sick.

Izuku then catches her, telling her he could tell the trajectory of the bullet heading toward Kai was slightly off, and that if she really was on All For One's side, she would've just shot Izuku from the beginning rather than give him the chance to surrender. He tells her that because she has seen the darkness, she should know how to bring back the light, asking her to join him as he believes she still has the soul of a Hero.

Seeing his bloody arm grabbing ahold of her, and hearing Izuku's words, causes Kaina Tsutsumi to smile as she is about to call him the real deal. However, she suddenly explodes, as unaware to them, All For One had placed a self-destruct Quirk within her in case she failed her mission or tried to turn on him. As her charred unconscious body starts to fall again, Hawks arrives to try and save her.

After being once again rescued by Izuku, Hawks cries out to his predecessor, yelling at her for letting All For One manipulate her, and that now she must know they can't give up, begging her to tell them what she knows, still calling her a Hero. As she starts to wake up and notice Hawks, Kaina Tsutsumi thinks about how the Safety Commission must've also made him do bad things too, and can't understand how he and Izuku still have that heroic fire in their eyes.

Regaining her composure as they land on the ground, she manages to scream out that her orders were to deliver Izuku to a mansion in Haibori Woods within two months, as well as revealing there are others All For One called upon in Tartarus as well. She weakly tells Hawks her heart couldn't bear the dark side and asking how he can still be hopeful, as he responds that he's optimistic to a fault. Kaina Tsutsumi passes out as she is apprehended, alongside Kai, by an arrived Endeavor and All Might, as they prepare to take her to receive medical aid, while Izuku watches over her body, festering over rage towards All For One.