Personally it's hard to recall disappointing characters because more often than not they are just forgettable. It's difficult to recall forgettable characters. I had to give this a lot of thought.
Darth Maul for an obvious answer I guess.
FPLOON said:
OT: At first, I thought the character Thomas from Regular Show was pretty disappointing after his first appearance... He did get two or three decent episodes revolved around him, but I kinda still want to know more about this intern... besides the fact that he's the butt of all the jokes geared towards him...
I agree with this. The character adds absolutely noting to the show and he is a dreadfully dull strait man character to boot. That's especially problematic for Regular Show because its such a dull series to begin with.
Looking at your avatar FPLOON, My Little Pony: Friendship is Magic's King Sombra was pretty bad.
A lot of characters in Bioshock: Infinite, mainly the antagonists. The Songbird immediately comes to mind but Daisy Fitzory was another one in a way. The same thing for the Handymen if they were suppose to be comparable to the Big Daddies.