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.
Logan Thackeray was extremely hyped up in Guild Wars 2. His ancestors in GW1 (one of whom was a major character) had an entire questline setting this up. He was the main character of a prequel novel. He was THE human main character.
And then the game came out and people hated him. Actually they hated him in part because of the prequel novel. Lots of memes. “For sale, Logan’s sword, never used, only dropped once.” And now the game has relegated him to a faction leader/desk job role.
(I always liked him though. Unpopular opinion).