There is an absolutely overpowered character, and the story creates compelling reasons why they're not around more often

Gildarts (Fairy Tale). Compared to everyone else for much of the series, Gildarts is and oppressively strong mage that uses 'crash' magic, to the point that a city is made to rearrange itself into a straight line when Gildarts comes back to town, just because if Gildarts bumps into a wall, it'll crumble or collapse or explode outward. With him being so strong, Fairy Tale often has Gildarts off on his own quests during the series, and only crossing paths with the protagonists occasionally.

Kenpachi Zaraki (Bleach): Kenpachi could solve most issues the protagonists run into, at least up until Thousand-Year Blood War, but he just doesn't care. He's only really interested in fighting 'worthy' opponents, including the protagonist. He's also a very poor fighter, but just completely outmatches his opponent in raw strength, which is another trope I love, but I'll probably make a separate post for that.