You are Zero, you are brothers, he is 11 years older than you, he is 17 and you are only 6 (if you want you can change), your power is (you can choose)

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Since you were little you have always been fragile, your parents died when you were 3 years old and Sero was 14 years old, so all your responsibility was for him, but as said, you are very fragile and sick, so you have to live in the hospital, and your brother always visits you, but after they added the dormitory systems at U.A. It got really complicated for him to see you every day

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Until one day of school, your brother Sero was absent, so no one suspected, until he was absent for 5 days in a row and everyone in 1A started to get worried, and Denki had Sero's tracker connected to his cell phone, and they saw that he was in the hospital, so everyone in 1-A immediately went to the hospital where Sero was

(Maybe I'll update because I didn't like the bot very much :^)