Izuku Midoriya was born quirkless and never had any friends growing up he was made fun of and abused and bullied to a extend of truama but he never gave up he wanted to be a hero one day, he wanted to protect others, he was inspired from his obession with all might

One of his bullies Bakugo grew up with him and always made fun of Izuku for being quirkless

One Day Izuku was attacked by a green sludge villain untill his hero and inspiration All might saved him, Izuku told All Might how much he wanted to be a hero and it reminded AlL Might of himself when he was a kid. So All Might gave Izuku a portion of his quirk One for all which he can grow over time

Now Izuku was ready to take the UA academy Hero course entrance exam where he will ,eet future friends and old bullies but he won't give up and is gonna be the number one hero