Your NPC is more than one NPC
A.k.a the multi-phase boss fight
Build more than one stat block for the character. This allows you to make honest and consistent rolls for the current situation. The rest is just the usual descriptions and roleplay.
There's already precedent in the DnD canon: there are legends that Bahamut, the Platinum Dragon, sometimes explores the mortal plane on foot, pretending to be an old man in plain garb accompanied by seven canaries.
The nice thing about NPCs is that they don't have to play by the same rules as player characters. There's a whole host of tropes about kings, princesses, legendary warriors and the like hidden among the common folk, sometimes for years. You can create multiple stat blocks, just like for multi-phase bosses, that represent how much the character is holding back. This represents a practiced persona, not something attempted on short notice, which is why its consistent.
Vash is the village pushover. He's always losing wrestling matches with the kids. He's a nice enough guy and he's always helpful to anyone who needs it, but he really couldn't win against those kids, according to the stats here labeled 'Vash, Village Pushover'. AC 9, HP 5, Attack -1. He stood no chance once those ruffians roll into town. A single shot took him out while he defended the lady who runs the restaurant. Strange that he's still breathing, though.
Until he's given no choice but to fight. Now he's 'Vash, Mysterious Gunman'. This stat block says AC 15, HP 60, Attack +7 even Acrobatics Expertise. Too much for some random bandits to handle safely, but surely within the normal realm or possibility, right?
Then the strange happens. A mysterious heavy hitter is leveling buildings, and our pushover is almost not recognizable. 'Vash the Stampede' has AC 22, HP 300, Speed 50. It's a good thing that this NPC has his own story that only temporarily intersects with our player characters. Either that or events resolve so that Vash can go back to leading a peaceful life losing wrestling matches against kids. At least nobody died.