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.
Tyr from GoW Ragnarok. He was heavily present in the marketing, hyped up all throughout GoW 2018, and he ended up being a very obvious fake-out. I wasn’t surprised at all by the time he killed Brok. And it was such a lame thing to do to the character. I wanted to see the real Tyr. Instead he feels like how Luke Skywalker felt in the new SW trilogy. There just to say he was there really. Ugh I get annoyed just thinking about how stupid I thought this plot point was.