Terrence is a recurring villain in Foster's Home For Imaginary Friends and is arguably the series' main antagonist - he is the 13-year old brother of Mac, the series protagonist, and like many older brothers enjoys picking on his little brother - however Terrence goes beyond the usual antics of an older brother, being a downright bully towards Mac on many occasions: in fact he seems to make it his main priority in life to make Mac as miserable as possible. He even later on gains his own imaginary friend is named Red. His desire to torment his younger brother for his own amusement has led him to even deliberately place Mac in life threatning situations without regard for his safety though he always recieves his karmic punishment for his malicious actions.
However like many bullies in cartoons Terrence is extremely dumb and as a result is easily outsmarted - this doesn't seem to stop Terrence from trying new ways to do bad things to Mac and his friends, not seeming to learn much from his many mistakes in the past.
In the shows first ever episode. He appears as one of the two main antagonists throughout the whole episode; serving as the main, later secondary antagonist throughout the whole episode. For he was the primary threat at first until he became a minion for the true main villain throughout the rest of the episode.