New Ade from a planet called 'dirt'
Quirkless and weak Quirked kids rounded up, Izuku among them, to be used as guinea pigs to test out an ancient device that had been unearthed. Those who took them figure they won't be missed.
It sent them through time and space onto another world… fortunately, one with Mandalorians before the schism and the wiping out of most of them.
They are taken and raised to be Mando'a.
Izuku is nominally their leader as he was the one who pulled himself together enough to both try and defend them and then to try and make contact and figure things out.
Even at thirteen, he was making a name as an analyst and strategist… also for getting into everything and anything and absorbing all he could like a sponge.
At fifteen, when word came that the place where they came through was showing signs of activity again, he was permitted to take a small strike group and attempt to return.
They manage to leap through and bring the younglings back with them. To where they will be loved and valued and raised to be people rather than left to rot.
(Not all that first group wished to return. The universe, even as cruel as it was, was far kinder to them than their homeworld… they had yet to actually track it through unknown space. They were actually better off.)
Arse kickery promptly commenced.
They were better trained, had better weapons and better tech… they were also innately more ruthless.
They also had the Mandalorian habit (baring the Death Watch) of loathing abuse of younglings and adopting any that strayed close enough that didn’t have good home lives.
They were a shock to the systems of Earth's governments.
As at least half of their number was made up of alien species but no one would have known it. After all, Quirk Mutations came in a wide array of forms.
It was nearly twenty years since they’d been gone local time, which was a shock. Those who had family who cared well, most of their family members had moved on already.
They rallied fast enough. Mandalorians had to be adaptable to a degree.
Their clan and battle brothers were there for them and it was enough.