Several characters have met their demise in the MHA manga, but it's a bit complex to list them all precisely. Some of the notable ones include [mention specific characters if known].
I think it's hard to say for sure. There are a few characters who could potentially be the traitor, and it's all part of the mystery and suspense of the story.
Deku is quite popular. His journey to become a great hero can be easily translated into a gaming context, like him leveling up his skills. Bakugo is also a favorite. His brash personality and powerful quirk make for exciting scenarios in fanfictions.
Winston is the main character who experiences a form of death. After his time in Room 101, he no longer has the same will to resist. He gives up on his beliefs and becomes a tool of the Party. And we can assume that many people who were once rebels or had different ideas in that totalitarian society were killed off. The Party doesn't tolerate any opposition, so anyone who goes against them is likely to face death.
Izuku Midoriya is really popular. His journey from being a quirkless boy to a powerful hero - in - training is very inspiring. Katsuki Bakugo is also popular. His explosive personality and strong power make him stand out. And Ochaco Uraraka with her cute appearance and kind heart is liked by many.
I don't know for sure as it depends on the specific fanfic. It could be a main character from the original My Hero Academia series who is put in a new and difficult situation that makes them 'broken but not defeated'.
The Boys comics have a few deaths. Some are villains, and some are characters you might not expect. The way these deaths unfold and their consequences shape the course of the story in interesting ways.
In Pulp Fiction, various characters meet their demise. For instance, Brett is killed by Jules and Vincent. Also, Butch's opponent in the boxing match doesn't survive.