*Slight trigger warning for suicide mentions and All Might slander*

Leinei gave a basic explanation of her quirk, as much as she could to her understanding, in Chapter Twelve, talking to Katsuki. However, for the next chapter where they're developing their ultimate moves, a little more information is required.

So as she said, her quirk works by redirecting the neurons in one's brain that parrot and reflect emotions back at themselves and the person. That's how it worked, at first. Like I said in the first Extra bits, her quirk is a mix of my RWBY OC's semblance Fear Monger(basically Relive but not with the special perks) and Lie Ren(also from RWBY)'s semblance to mask emotions.

Leinei's quirk targets a person's limbic system, the body's emotional response and creation unit basically. In case you don't know(I didn't even know until I started doing more research for her quirk because I wanted a legit scientific explanation for her quirk because I'm weird about shit like that), the limbic system is made up of the hypothalamus(controls emotional response, hormone release, sexual responses, and regulates body temperature), the hippocampus(perserves and retrieves memories), amygdala(coordinates emotional response to external stimuli and plays a big factor in fear and anger), and the limbic cortex, which is the structure made of the cingulate gyrus and the parahippocampal gyrus(impacts mood, motivation and judgement).

Her quirk specifically targets the hippocampus and amygdala, while poking at the limbic cortex and hypothalamus, and the body's response to external stimuli via the nervous system. As seen during the prologue, her quirk can impair one's judgment following the initial hit, as seen when those three students committed suicide. The initial hit, if actually seen from the victim's POV, is more of them being trapped in a memory, or a memory of a fear, and reliving it to varying degrees.

So for example. Say she used her quirk on Izuku(not saying she will. Just an example). I believe there are three core bad memories he could relive(as far as this AU is concerned, I will not use manga for spoiler reasons), and that would be when he was five or six being told he will never have a quirk, having All Might tell him he would never be a hero, and failing to save Katsuki. Maybe we can throw in Nighteye dying, but I feel that's more of a sad traumatic than a true fear. So out of those memories, let's rank them by how they impacted him, negatively. I feel number one would be All Might telling him he would never be a hero(not because I hate All Might, because I do for varying reasons, mostly how he interacts with Izuku, but I can write a whole essay on that), because that's just crushing to sit there and hear your idol tell you that you will never be good enough. From there, not being able to save Katsuki or Nighteye, which reinforces the initial reason, and then finally him being told he would never have a quirk.

So, we would use the number one fear or memory of a fear, and Izuku would relive that rooftop moment, having it get worse and worse in his head. Going from "You need a quirk to be a hero" to All Might berating him and cruelly mocking him until Izuku either shuts down, or chooses to take a short walk and a long drop. So the initial hit, and how the person copes with it, or how mentally and emotionally sound they are, generates that secondary response. For the students at Shiketsu, they went the suicide route.

Most of the time, the body chooses that secondary route, by shutting off the ability to feel external and environmental stimuli and essentially "numbing" them as a defense mechanism, as seen when the students during the Sports Festival in Chapter Seven when they passed out after being hit with her quirk for a millisecond. However, sometimes the body can't numb itself quick enough and releases too much adrenaline and cortisol, which is what gives us that fight or flight factor in extreme situations. In doing so, that is what causes older people hit with her quirk to have fatal medical conditions, such as heart attacks(Shiketsu teachers) and strokes(Pro Hero Galliant).

HOWEVER! Our beloved heroine can do more than make someone relive their worst memories! As we've seen, she can use a person's emotions to track them down, using the emotional imprint they leave behind. Like an empath bloodhound(casually just putting that away to use later). She also describes her passive ability of her quirk as an emotional radio tower, which is right and wrong. She is very good at picking up everyone's emotions around her, and she can't turn it off. She has since learned to block most of it, hence why she acts differently now, but she still can pick up personality traits of those she's around often, if those emotions and traits are stronger than those limits she has. However again!

Leinei is able to do more with her quirk than we know of. I already have her ults planned. They're pretty fucking cool, and I love them. She may have brainstormed names with Tokoyami, but that's all fine. Thank you for reading this science lesson, and you are all awesome! I hope this clarifies any questions you may have had on her quirk, and helps to make sure reading is smoother, since I'm a bit of a dummy when it comes to continuity at times.

Sincerely, Luna Sylvies

Thank you for reading!