Daria/Tom in Daria
. The introduction of Tom in this 90s MTV cartoon has always been controversial among fans. I never cared much for him - he's bland and doesn't have any distinctive character traits of his own. He first dated Daria's (only) pal Jane, causing tension between the two, and then cheated on Jane with Daria. An attempt at character development that completely changed the direction and tone of the show, and not for the better; it became too melodramatic and, a lot of the time, not as funny.
Kim/Ron in Kim Possible.
One thing I really liked about this show was that Kim and Ron were very close friends - and that was that. I actually enjoyed some aspects of their developing romance at the time but, in retrospect, it would have been better to have kept it the way it was. And why the writers decided to "ship" Drakken and Shego out of nowhere in the last episode is truly beyond me.