A character who was pretty awesome when they showed up. Usually they weren't meant to stick around but everyone liked them so much the author/company decided to keep them around, but unfortunately they went the wrong direction with this character.
Personally Vegeta is what made me think of this.
In dragon ball Z when Vegeta first appears he's pretty funny very patient and pretty smart. The interactions between him and Napa were amazing. Then Goku shows up and kicks his ass [though he really had to work for it]. And Vegeta goes to planet Namek. That's when he progressively goes on a downhill slide and eventually ends up crying and whining every saga.
So, at the start he was pretty cool. IMO.