Honkai: Star Rail: The Hero Makes an Entrance

Ronan, from another world, is fighting the Abyss when he crosses dimensions and arrives in the Honkai: Star Rail universe. Burdened by debt and unable to return home for the time being, he decides to fulfill his childhood dream here.

That is, to become a hero.

“Stay behind me. I'll save you from tragedy.”

[A heavily modified plot]

The timeline in Honkai: Star Rail is too vague, and the ages of many characters and the events they've experienced aren't explained in detail, so I'm going with my own version.