Yuck is an Evil Level 5 Woo Foo Warrior. He is an entity formed from Yin's obsession with control and Yang's aggression. He wants nothing more than to become the most powerful Woo Foo Knight ever, and to do that, he needs to eliminate Yin and Yang plus Master Yo. In the episode Upstanding Yuck, he became good, but Yin and Yang didn't believe him; in the end, Yuck turned evil again after Yin and Yang attacked him many times. Despite being half female, Yuck is often considered a "he" and lacks eyelashes, much like Yin and Yang's combined Aura.
Yuck's general body shape is like Yin and Yang's: Short, round head and floating ears. His fur is light green and shaggy, his eyes are amber colored and his nose is purple instead of pink. His teeth are sharp and he has some spots - maybe dirt- on his face. He wears the same dojo uniform as Yin and Yang with the difference that it's torn at the bottom. He is glaring almost all the time.
After absorbing multiple Fooplicates in Imperfect Fooplicates, he gained the ability to become tall and muscular. His shirt disappears and he wears long purple pants. He retains this appearance for most of Personality Problem.
In Upstanding Yuck, after cleaning himself up, Yuck's fur is shown to be a minty green. It's no longer shaggy, but distorted and falls into single strands at the back of his head. For a short period of time he wears a black hoodie. Underneath he wears a clean and fixed dojo shirt. That is, of course, until he becomes evil again, due to Yin and Yang retaining their aggression against him.
As he was originally created as a combination of Yin and Yang's negative traits, he has inherited their bossiness and aggression, respectively. He is short-tempered and has a sense of entitlement, as seen in Yin! Yang! Yuck!, when he expresses a desire to kill Master Yo after the latter refuses to train him multiple times.
He is evil, being driven by nothing besides negative qualities. He doesn't care for hygiene, manners or morals. Additionally, he has proven himself to be quite intelligent and manipulative, tricking Yin and Yang a couple of times into playing into his hands. As seen in Imperfect Fooplicates, he loves not only being in charge, but also being complimented and respected by others; by doing so, Yin was able to distract him long enough to steal a Fooplication Charm from him.
Despite being a being created out of nothing but bad qualities, there has been some indication that there might more to him than just evil. In Falling Yin Love, he seems quite happy when Yin says she wants to be with him despite his true identity. In Upstanding Yuck, thanks to some outside interference, he manages to overcome his bad personality. During his time as a good person, he is friendly, kind, helpful and very polite.
Since being created, Yuck has been temporarily destroyed multiple times.
- Yin! Yang! Yuck!: The twins use each others "bad traits" to battle him. They manage to defeat him, causing their bad qualities to be returned to them. But only his physical form is destroyed, his spirit remains alive.
- Falling Yin Love: After being defeated the first time, Yuck is found by Flaviour and the Night Master. They built him a robot suit that can absorb Woo Foo which he uses to trick Yin into dating him so he can consume her energy over touch. Eventually Yuck absorbs enough Woo Foo to become real and powerful again. Yin tricks him into thinking she likes him while she wears the hands of the robot as gloves. This causes Yuck to lower his guard which Yin uses to embrace him and absorb his new power. They defeat him, but don't destroy him.
- Voyage To The Center Of The Yo: Yuck, now working for the Night Master, hides inside Master Yo so he can grab the Toilet Brush of Illumination when Master Yo gets it but, Yin and Yang spot Yuck entering Yo. They follow him inside, and after multiple fights they eventually trap him inside Master Yo's ear where Yin destroys Yuck's shrinking belt, causing him to shrink infinitely, thus shrinking out of existence.
- Imperfect Fooplicates: Yin and Yang's first fooplicates accidentally bring him back by trying to fooplicate themselves. The power ray coming from this action hits a mirror, the fooplicates melt into each other and create Yuck. In the end of the episode Yuck consumes the dust that 'de-fooplicated' fooplicates turn into, which gives him power and makes him insanely strong. However he gets a bad case of the hiccups and ends up punching himself out, unable to control his new powers. He is defeated but not destroyed.
- Camp Magic Pants: Yuck also comes back in "Camp Magic Pants", hiding in the body of the headmaster of the magic camp. After Yin becomes consumed with naughtiness, she touches the Golden Pants (a statue in the camp) and he reveals himself. He uses the Magic Pants to try to destroy Yin and Yang but with the power of disco the twins force the pair of pants to dance, eventually sending Yuck flying, leaving him defeated.
- Upstanding Yuck: Upon his next return Yuck is a good person. The twins distrust him despite even Master Yo saying they should give him a chance. They keep misunderstanding his actions as crimes and beat him or ambush him. He never fights back, having changed. In the end they give him such a harsh (and undeserved) beating that he becomes evil again. They have a rather short fight. Yuck destroys a part of the town and simply leaves after taking his donated money back from a kid.
- Personality Problem: His next and last appearance on the show is when it is revealed that there is a crown that automatically makes you rule the hometown of Yin and Yang. Trying to steal it, Yuck distracts Yin and Yang by switching their auras (and thus personalities). They end up figuring this out and manage to switch back to their original selves and defeat (but not destroy) Yuck with their combined aura. As he has not been destroyed, it is likely that he would have appeared in Season 3 (if there had been one).
Being a creation of Yin and Yang, born from their traits, Yuck has all their abilities, plus his own Foonado, referred to as "Yuckicane". He has also been shown to use Energy Rays, Levitation, Foo Field, Fists of Fire, Foo-portate, Foo Orbs, Paws of Power, Bamboo Weaponry, Aura Swap, and Foo-Plicate (with the Foo-Plication Charm). He also seems to be able to possess people, as seen in Camp Magic Pants. He can also change into his more muscular form at will.
As Yin and Yang learn new abilities, it is very likely that Yuck is capable of those abilities as well. He also has enhanced strength, intelligence, agility, and is highly resistant to damage.
- Yang: Yang and Yuck are foes. They are shown hating each other most of the time they interact (excluding Yuck's actions during "Upstanding Yuck"). Yang's anger at him seems to increase after the events of "Falling Yin Love".
- Yin: Yin and Yuck are enemies. Throughout the series, he appears to have more of a relationship with her than Yang, since he tricks her most of the time ("Falling Yin Love", "Camp Magic Pants") but never focuses his schemes on Yang. At the end of "Falling Yin Love" he appears quite happy when she lies about liking his true self. Furthermore, he dislikes his girl part, which he inherited from Yin. This may partially stem from Yang's dislike of girly things, as he was made of the twos negative properties.
- Yo: While Yuck and Yo dislike each other, in the beginning of his existence Yuck wanted Yo to train him. When Yuck becomes a good person, Yo is the only Woo Foo to actually believe in his change and trust him.
- Night Master: During the first season, Yuck starts serving the Night Master somewhere between the incidents of "Yin! Yang! Yuck!" and "Falling Yin Love".
- Yuck technically has no gender due to being a mix of the twins, but is normally referred to with masculine pronouns.
- The fandom often calls his "Good" side Oliver.