Dani Takami
Mutant quirks are rare in the My Hero Academia universe, especially if they're inherited. So, when Dani was born to parents Hawks and Sasha Wright, he was a quite literal miracle; even if Sasha didn't see it.
Sasha saw him as a successful dual-quirk project; one that perfectly combined both her and Hawks' quirks, Poison Angel and Fierce Wings. As such, she abused both Dani and Hawks, all in his childhood and before he was even born. Sasha would outright physically and emotionally abuse, as well as rape, both him and his father, though Hawks' suffering had gone on for 15 months longer than Dani's.
That still didn't stop her from using Fierce Wings against Dani. She had forcibly ripped out a good chunk of his feathers from his wings and used one of the longest, and therefore strongest, feathers to cut deep into his left flank when he was just a defenseless 8 year old. In result, the wound never healed properly; it started to die and rot away on him while he was still alive, and he had to get it completely removed. He now has a large and denting scar where the wound used to be and the operation happened, leaving his left flank completely missing and a small, yet noticeable, portion of his large intestine exposed. He now wears a prosthetic to cover it up, as he's come to completely hate the scar.
As a result of all the abuse both he and Hawks shared, Dani took it upon himself to stop their shared suffering, and killed Sasha when he was only 12 by decapitating her with his raccoon tail. He knew that he'd be the only person who was strong enough in his family - or even the world - that could take on the challenge, and he wasn't (and still isn't) the type of person to back down from an opportunity to free a person, even himself, from a bad situation. That's why he's the top 10 hero of Japan, 'Vamp,' at age 16.
The abuse didn't stop at the flank incident, though. Dani was also thrown, and therefore slammed, against the wall, as well as manipulated, gaslit, and isolated on a regular basis. As a result of being so isolated, he didn't have a social life until middle school, where he got introduced to Kirishima, Aoyama, Tamaki, and Daniel &, as a result, his current friend group.
Hawks did make it a lot easier for him to go through, even if he was also being physically abused, raped, manipulated, and gaslit at the same time as him. He felt seen and cared for by his dad, like someone was actually there for him and a good parent.. and, therefore, his absolute best friend. Hawks knew what it felt like to go through the exact same thing as his son, and he was - and still is - the absolute best parent that Dani could ever ask the universe for…
Even if Dani was almost completely silent and reacting with defense, fear, and uncertainty when he met and was surrounded with people in middle school, Kirishima, Daniel, Aoyama, and his friend group still treated him like any normal person. They were open to waiting to earn his trust, being themselves around him, and ending up being his friends; which they did. Not to mention, Dani and Tamaki knew of each other, but didn't actually know each other, for a while. Then, Daniel officially introduced Dani to Tamaki in the summer between grades 8 and 9 - and they both immediately clicked with each other. Tamaki felt as if he'd finally found someone who was willing to sit there and be patient with, as well as fully understand, him, and Dani did too.. and they both just instantly felt completely at ease and comfortable with each other right off the bat; something that was unusual to see with the both of them.