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, Shredder in 2012 Teenage Mutant Ninja Turtles version, Sarevok Anchev in Baldur's Gate, Gantu in Lilo & Stitch franchise, Goro in Mortal Kombat film, Milleous in Ben 10 franchise, Trigonin Teen Titans, Trigon's 2013 incarnation in Teen Titans Go! and Rhombulus in Star vs The Forces of Evil.
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.