Yuck is an evil Level 5 Woo Foo Warrior and a recurring enemy in the TV series Yin Yang Yo!
This entity was formed from Yin's obsession with control and Yang's aggression. As a repulsive, bad-tempered mutant rabbit, Yuck wants nothing more than to become the most powerful Woo Foo Knight ever. The one way he can accomplish this is by eliminating Master Yo and his creators, Yin and Yang. Despite being half female, Yuck predominantly sports male gender characteristics and lacks eyelashes, much like Yin and Yang's combined Aura.
Yuck's first appearance was the episode "Yin Yang Yuck". When Yin and Yang started fighting each other over the best way to attack, they ended up using Woo Foo powers on each other. All of the twins' negative traits were forced out of their bodies, merging together and forming Yuck. While Yang and Yin had become significantly nicer (particularly to each other), it was clear that Yuck was all bad, as he was attacking bystanders at random.
Yuck showed up at the dojo and demanded that Master Yo train him to be a Woo Foo Knight. Instead, Master Yo kicked him out. Yuck angrily vowed that one way or another, he would train to be a Woo Foo Knight. He eventually engaged in a fight with the panda and beat the knowledge out of him. When the rabbit twins tricked Yuck into using a Woo Foo attack on himself, Yuck was drained of his energies and reduced to nothing more than a shadowy visage.
Yuck was found by the Night Master after being vacuumed up by the master's right hand man Flaviour. In return for his services, the Night Master gave Yuck a dreamy robot body with specialized hands that could drain enough Woo Foo energy to give him a real body.