Ok this request was really hard for me because I'm terrible when it comes to making stories.
I think Reigen would either have a very weak/useless quirk (like changing his hair color) or he would be quirkless.
You can probably guess what these two are saying. Sorry. I got lazy :")
Now watch me try to write him a backstory at 1am ;) (but to be honest, it wouldn't be much better if i had written this at 10 am lol)
Let's go with quirkless because angst n plot! Also it just fits better, i think.
He has been lying to everybody that he has a quirk since middle school. A quirk like Silver tongue would be fitting but make people more wary of him, so he convinced people he has something else, sth cool. He can probably do all types of magic tricks n then said that's his quirk. When he started Spirits and Such, he obviously also started lying to everybody that he has a sort of psychic quirk. (Imagine if he meets somebody from school and now has to convince them that his quirk has changed or he was hiding his "real" quirk because his psychic ability was so powerful that would have been too dangerous for himself to reveal it. What if he had gotten kidnapped by villains!?)
So all the stuffs from MP100 still happened with little changes. Because power exists, why not ghosts? And power specifically linked to ghosts? Or quirks that is similar to psychic abilities. Or maybe quirks are actually a result of some supernatural incident, pretty sure quirks just kinda appeared one day in mha so that should works.
Now how did he got into all this anyways? I think it started out as self defense, the places he frequents and the people he deals with tend to be a bit shady after all. Then it became dealing with small criminals like purse snatchers or bullies. He uses his words most of the time and only use violence as a last resort. (Because he isn't THAT good at fighting) Everything was going fine for a while, people even gave him gifts to show their gratitude, but one time he unknowingly threw somebody from a small gang in jail. The gang confronted him. He probably bullshitted his way out of it but that was a close call. (He started training seriously after that, mostly still for self-defense) He really should hide his identity whenever he decide to play hero, but it's not like he can plan when he stumble across a mugging... He decided just change his tie and wear a mask lol
That's how he unintentionally became the local vigilante.
(Ok kinda random, and idk if it works well or not since i last watched mp100 a year ago, but at first Reigen just says whatever put him in the best situation, he only thinks about himself. After he met mob though, an increasing number of his confrontations became counseling sections.)
Ok so like i said, all the stuffs form mp100 still happened, with little change. And that was what thrusted him into the spot light.
But when exactly? How? Also why haven't the police caught him? While it's technically not illegal because he is quirkless, the police don't know that (yet). I couldn't come up with anything so naturally i turned to fanfics for inspiration. And Balanced at the hilt works really really well. Just copy paste it here, it's exactly what i want. There's even a reason why he decided to go beyond accidental heroism and actively go on patrol (there are very few heros, maybe none in that area because some big incident happened 20 years ago, which makes it a paradise for villains) He still try to only focus on small criminals of course.
Stuffs happens, now he is officially an underground hero and a teacher at Yuuei. His quirklessness is still a secret. Only a few know the truth and i think you can guess who. The officials all agree it would be more beneficial if they keep that hidden too (or he just convinced them). Spirit and Such is still opened, now run by Serizawa and Dimple, the kids stop by to help out sometimes too.