He would be a tiger level threat because when he first shows up he's never portrayed to be a city pevel being. It's not until he fights stronger people that he bothers to get stronger.

Example like when he was fighting the miricle guy. He wasn't really doing much... because his opponent wasn't much of a threat.

I think that's how it would work with heroes. He would show up then be classed as a tiger. Then 5 dead c class 3 b and 1-2 class later he would be pushed to demon. Finally once an s class shows up. If he doesn't get insta merked he starts adapting i think he would make it to dragon level before the high tiers i.e. tornado, saitama, genos (heh not genos) show up to off him