Yumyulack is an alien called a Shlorpian who originates from a planet called Shlorp. However, ever since an asteroid destroyed his home planet, he, alongside his family, Terry, Korvo, and Jesse, have to scatter to a home planet. They wind up picking Planet Earth in hopes where they could settle, and make a decent living. They are, for the most part, not well-adjusted to Earth's customs. Yumyulack and his family try to overcome the challenge of being immigrants to a relatively unknown planet. Because of their inexperience, the Shlorpian family gets themselves in wacky hi-jinx.
Yumyulack is a Shlorpian, a species of alien which originates from a planet called Shlorp. He has light-blue skin, with bluish-gray speckles on his forehead. Yumyulack is usually seen wearing a grayish-tan vest with white shoulder pads and a long-sleeved shirt that comes down to his elbows. He wears a utility belt with multiple pockets and a silvery belt buckle.
Yumyulack is very cunning, crabby, sly, and likes pulling pranks. He claims that one of the reasons he's perceptive is because he's a scientist and loves doing "sci-fi shit".
Yumyulack claims that he used to be a bounty hunter back on planet Shlorp, which could explain why he is able to make quick work out of the bar full of Neo-Nazis and know how to use advanced tech like shrinking rays.
Yumyulack has a knack for outer-limits pranks and torturing people. He hijacks his school's Mac computers in order to program his own artificial intelligence into them as an elaborate joke, puts people in pods, shrinks his classmate to pour Diet Coke in her exposed brain, and locomotives her all because she teased both him and Jesse. He also harbours people who he shrunk using a shrink ray in a place called "The Wall" and subjects them to torture. He claims to be doing these pranks to "study the human condition". After Korvo and Terry advise him not to torture fellow classmates, he complies, but still tests on and experiments with adults. Yumyulack says does this because they "love it" although that much is debatable, as they are constantly seen in pain, and looking for methods to escape.
He tends to be generally annoyed with his replicant sister Jesse. Unlike Jesse, he has little to no restraint in hurting humans, as he finds great pleasure in torturing those who wronged him or simply get at his nerves. Jesse is opposed to this, as she wants to try to understand and fit into human culture. Despite their differences regarding Earth and humanity, deep down he secretly cares for her.
Yumyulack and Korvo generally have a good relationship.
He describes Korvo to be "mean, but being mean is tight" implying that he has respect for Korvo.
Although he is usually at odds with his replicant sibling in regards to their views on humanity, deep down he cares for her.
His defensive nature towards Jesse is shown in "The Unstable Grey Hole", where Jesse discovers that Yumyulack is torturing too many innocent people. In turn, Jesse tries to expose Yumyulack to the good side of humanity by convincing them to like Yumyulack. Yumyulack doesn't budge, and decides to show Jesse that humanity has a bad side as well by leading her to a bar of Neo-Nazis. Shortly after, they get ambushed by the Neo-Nazis, causing Yumyulack's automated suit to activate into defence mode. Yumyulack starts committing bloodshed against the Neo Nazis in order to protect him and Jesse. Although he claims that he wasn't in control of the suit, he still takes them out with lethal force and winds up saving her life, in which she thanks him for. They both realize that humanity is not as black and white as they thought - and that there are both good and bad sides to it.
His strongest instance of insecurity and his desire to be liked by Jesse is shown in "Retrace-Your-Step-Alizer". This episode shows he has no regards for Jesse's privacy, even when Aisha tries to discourage him, commenting "That's why it's going to be awesome" before he overrides the privacy protocol in the Pretend-o-Deck with the password "Panera Bread with 3's for the E's". He is offended and disgusted when he discovers that Jesse has an alternate nicer version of himself called "Funyulack" in the Pretend-o-Deck who likes to play dress-up and knows how to braid and bake bread. Funyulack reveals that Jesse thinks "[Yumyulack] [is] a dick" and brings up recordings of Jesse's previous statements as evidence, revealing that Yumyulack once tried to trade her to a witch for a PS4 and that he makes fun of the way she walks and breathes. He spends the rest of his episode rehabilitating his impulse to be mean to Jesse with Funyulack.