Deku

Izuku's first reaction would be paralysis and disbelief. His analytical mind would try to find a logical explanation, as he has unwavering faith in his friends. Seeing someone he loves killed by an ally would cause an immediate emotional "short circuit."

He would try to understand the traitor's pain, but the loss of a loved one would create a permanent scar.
Unlike a typical hero, he wouldn't just seek imprisonment, but a confrontation that would force him to choose between justice and revenge.
He would neglect his own life (not eating, not sleeping) in order to hunt down the traitor.
His worldview would be shattered, turning him into a much more cynical and cold hero, focused solely on preventing something like this from happening again.

"Terrifying" Altruism: Its most striking characteristic is the willingness to sacrifice oneself for others without hesitation. They often act on instinct to save someone, even when they have no real chance of success or when their own body is broken.
Determination and Resilience: Having grown up without a Quirk, he developed immense willpower. He is extremely diligent, studying combat tactics and analyzing heroes in his notebooks to compensate for his weaknesses.
Analytical and Observant: Deku has a habit of "muttering" while deeply analyzing the strengths and weaknesses of friends and enemies. This strategic intelligence is what makes him one of the most tactical characters in the series.
Evolution of Confidence: In the beginning, he was very insecure and withdrawn due to bullying. Throughout the story, he learns to transform the insult "Deku" into a hero name that means "I can do this," becoming a respected and confident leader.
Deep Empathy: Unlike many heroes, he tries to understand the pain and motivations of even the villains, seeking to save them from themselves instead of simply defeating them.
Did you know? His favorite hobby is collecting All Might merchandise and his favorite food is Katsudon (a bowl of rice with breaded pork).