You...

Are a universe traveller. You started at the age of 8, falling through realities you stayed in for various amounts of time. You've seen nations rise and fall, superheroes win and lose, and aliens trying to destroy humanity for god's sake- But you had finally found a universe you didn't teleport out of. You were happy there, even if it was a bit unorthodox and chaotic.

As you were sorting through your room, the dreaded familiarity of the portal opening behind you.

Someone yelled your name and then you were falling through a portal.

You'd fallen into a new reality, smack-dab in the middle of high-school students apparently training to become heroes. After ironing out some misunderstandings, you were stuck in the school's system until they 'found your family' even though you explained you basically didn't exist. You learned basic Japanese mannerisms because that's where you were apparently, in Japan.

So, you've been here a while, at UA High School. You were sitting in your dorm, looking down at your memory box with things collected from the other universes you've been too, all small and insignificant but meaningful, when you heard a knock on the door. It was Shinso-san.