Headcannon:

Ashe has bad nightmares from his past. Bakugo allows Ashe to stay with him on some nights to sleep since Ashe will actually sleep with him there. Bakugo is gentle with Ashe, so Ashe gets away with a lot more than most

Eventually, Ashe began to develop feelings for Katsuki. Katsuki was focused on his dream so Ashe chose to support that instead. It wasn't till Katsuki died in the war against AFO that Ashe decided to not hide it anymore. He confessed under the cherry blossom trees.

The class becomes protective of Ashe, seeing him as something that needs to be kept safe. Ashe's favorite food is Kimchi stir fry, made exclusively by Katsuki. He believes Katsuki is amazing and, given his lack of knowledge of the real world, believes the boasts Katsuki says.

Eventually, the three become close (as close as Katsuki and Izuku can be). Ashe tends to hang out more with Katsuki, always wanting to be around him. He basically becomes Katsuki's shadow, which the blonde likes even if he doesn't admit it

Ashe didn't know how to show emotions. So he was usually silent. Aizawa took the two most active members of the class, Izuku and Katsuki, to help Ashe relearn how to be in society. Izuku is more than happy with this but Katsuki is irritated.

Aizawa was feeding stray cats when he found Ashe. He took him to the hospital. Ashe slowly recovered, still barely talking. Aizawa took pity and adopted Ashe because of his situation. For his protection, Nezu allowed Ashe to be put into UA in class 1A to keep his quirk in check and him safe.

The black arms are from the experiments burning his skin. After a while, he became numb to the pain and was practically a mute. Dabi, being a newer member, felt pity for Ashe given his own past. He eventually helped Ashe get free, still leaving him for dead.

AFO took Ashe to a lab and the doctor where he actually met a young Tenko. Ashe was quirkless which made him perfect for a guinea pig for them to use for quirk creation. Over the course of 10 years, Ashe was tortured and experimented on, leading to horrific scarring on his back and malnourishment

Thank you so much!!!