Happy Birthday is the main antagonist of the Regular Show episode "Happy Birthday Song Contest".
He was voiced by Kevin Michael Richardson who voiced the Pig Sheriff in Samurai Jack.
Happy Birthday appears at the happy birthday song contest, after Mordecai and Rigbys song appear to have won. He sings the happy birthday song and showers confetti over the crowd, making them brainwashed and they start to walk away. Mordecai and Rigby then save them by singing their happy birthday song, freeing them. Happy Birthday then leaves, defeated.
Happy Birthday is a tall, lanky man with a birthday cake for a head, who wears a classy looking suit with a red cape.