For me it was Kai Leng in Mass Effect 3. In the intro cinematic he looked like a cool assassin that was sent after Shepard, but when you meet him he’s just a loser dweeb with insane plot armor and the power of anime on his side.
Another character for me is Alduin in Skyrim. The entire storyline hyped him up as The World Eater, that he will bring the apocalypse, but the when you do finally face him… I stg I’ve fought Draugr more fearsome.
That Joel guy from TLOU2. The ads and then the intro to the game made it seem like he was this super important dude who has gonna be your partner through the whole thing, a real ride or die, and instead he gets chumped out early and then doesn’t even matter after that.