Happy Birthday is the main antagonist of the Regular Show episode "Happy Birthday Song Contest". He is voiced by Kevin Michael Richardson who voiced Shredder (2012 TMNT).
When he heard that Farmer Jimmy was holding a contest to change his song, and that Mordecai and Rigby were going to sing a new original song, he interrupted the competition and proved his song was still good through the use of a hypnotic confetti and singing in a slow, crooner-like fashion. Mordecai and Rigby, however, beat him with their lighter and shortened version of the song. Everyone enjoyed it and they ended up winning, but he swears revenge by saying that everyone will grow tired of Mordecai and Rigby's new song one day, and that his song will be popular again.
Happy Birthday is a tall, lanky man with a Birthday Cake for a head, who wears a classy looking suit with a red cape.