Much like a TV series or a movie saga where you start the adventure with the protagonist and then eventually make it to the end, there's always a game character of the same principle that you sat through the events with and watched them become a prodigy with a major buff since the start of the game franchise.
Alternatively, there could be a specific side character or villain that you could consider to be the most overpowered character in gaming. As it pertains to bosses, I don't think it necessarily relates to how difficult that boss might of been, I'm moreso just going for in terms of ability and how we're supposed to perceive those characters. Some characters I'll jokingly classify them as overpowered because of the fact that they survive a fall or certain amount of damage.
Characters that show feats of overwhelming power such as Kratos from god of war, Asura from Asura's Wrath ,Sephiroth from Final Fantasy, Dante from Devil May Cry are all good examples in terms of abilities and feats, but other off the wall characters like something as simple as the stick guy you play in Fancy Pants Adventure or Sack Boy from Little Big Planet (Drawing something/creating anything out of thin air.), or Pac-Man and his wife for being able to successfully consume dots, fruits, and ghosts since the dawn of gaming.
An overpowered character can be labeled as such just purely on their capabilities, whether they have an explanation for their talents or not.