Izuku was a happy child until his father was killed by AFO and All Might defeated AFO for a time.

(Izuku's biological father)

After the fight with AFO All Might married Inko and he became Izuku's step father and he had a child with Inko, Izumi was her name(she was like a carbon copy of Izuku)

(Izumi)

Izuku was abused by All Might and neglected by Inko. He was also bullied by his and Izumi's friends because of one thing he had no control over... Being quirkless.

When Izuku was 13 years old he had enough and decided to leave.

Izuku: *leaving a letter for Izumi* I'm sorry Izumi... I just can't take it anymore.

Izuku then got some clothes and food to last him a month or two and he left.

The next day the Yagi family was awoken by loud crying from Izumi because she read the letter.

AM: ARE YOU OKAY IZUMI?!

Izumi: *crying* YOU ASSHOLE!!! YOU CAUSED THIS!!! I HATE YOU DAD!

Izumi then stormed off, leaving the letter an All Might read it.

(Beginning of letter)

         Dear Izumi,

I am certain you are reading this and I'm gone. Now don't be upset because I will be at UA just like we planned and I don't blame you for anything.

With love, Izuku

(End of letter)

All Might hated that Izuku had turned Izumi against him and he started looking for Izuku.

Inko was so ashamed of herself for neglecting her baby and she started stress eating making her short and chubby.

Meanwhile Izuku was running and was captured by Tomura Shigaraki to lure All Might.

Izuku was now chained to the ground.

Shigaraki: Now that I have you, I can break All Might... And sensei will be pleased with me.

AFO then walked in.

AFO: Yes indeed Tomura.

Izuku: *chuckles* Sorry to break it to ya, but he doesn't care about me... *coldly* I'm just a little punching bag to him.

AFO: So, will you help us young one?

Izuku: NEVER!!! YOU KILLED MY FATHER YOU BASTARD!!!!

Izuku was now thrashing around trying to get to AFO to somehow kill him but to no avail.

Izuku: *praying just below a whisper* God, please give me power anyone help me please...

Shigaraki: How amusing~? He's praying!

Mephisto: *in Izuku's mind* Do you want power to bring punishment to the wicked? If so say my name...

Izuku: MEPHISTOPHELES!!!!!!

Izuku then began to light ablaze and his head became a skull on fire as he let out a high pitched demonic screech.

AFO: Fuck this shit, we outta here. *teleports him and Shigaraki*

Izuku then went on a rampage of sinners.

The next day Izuku started training.