Monster Krumholtz is a purple Organic and is the show's deuteragonist.
Currently, little is known about Monster's past.
From what is already known, Monster is an Organic that has gone through life quite carefree and happy. At a young age, he befriended a Mechanical named Robot, despite warnings from others about Robot's dangerous inventions. The two have gone through school together as close friends (and often have had a scientist and assistant relationship), eventually winding up as roommates in an apartment. Monster seems to have a very good relationship with his family, though his younger sister, Globitha, is more interested in being with Robot than her older brother. Monster holds his father, Lev Krumholtz, in high regard for his accomplishments in the event known as Hornica. The only family member he is known to have "problems" with is his Uncle Kuffley, who often teases him but usually all in good fun and showing him love; it's nothing like the way Gart treats his younger brother, Robot. Regardless, Monster is a kind and loving soul who has tried to help others with their problems and cheering them on. Having not gone to college (as the college money was spent on Lev's horn care), Monster makes a living working in the Blinking Light Factory in the Quality Control department, alongside Robot.
Monster takes pride in being a very happy person, but once got depressed when he found someone he thought was happier than him. However, upon finding out that the happier guy, Perry, was really very miserable (his face gears are broken, forcing him to smile eternally), Monster cheered up and made a vow that he'd try to find a way to make Perry happy too. Monster is shown to be very naive and trusting, often doing what he thinks is right to help another person, even if that person might be lying to him. This is evident in the episode The Prince of Scamtown, where he kept giving all the money meant to pay bills to a guy he believed to be a prince. The so-called prince promised a wagon full of gold if Monster helped him out with minor money issues. Robot refused to believe that such a person existed until the end of the episode, finally trying to believe that the prince really existed. Much to his surprise, the prince was real, and like he promised, offered his wagon of gold. However, he asked for a few more dollars due to an incident at the bus station, causing Robot to send him away empty handed, thinking that it was still a scam. As it turns out, Monster's trust was in the right place, as Ogo suddenly came to them with the wagon of gold after handing the prince two dollars.
Along with Robot, Monster shares an apartment building with another neighbor, Ogo, whom has an obsession with both Robot and Monster. Despite not liking how clingy Ogo is, Monster still tries to be kind to the latter and not hurt his feelings. In Ogo's Birthday, it is confirmed that he truly does cherish Ogo as one of his friends, much to the outrage of Robot, despite both having been tricked into going to Ogo's party when it was never actually Ogo's birthday(his birthday not being for another few months).
Monster also thinks there is good in everyone, even Robot's older brother,Gart; and tries to be nice to all. However, there are times when his excessive kindness to others is often a problem, as shown in Anger Management: When a rude civilian began trampling all over his good nature, followed by the continuous mistake of not having any cream in his bacon coffee; Monster had obtained the habit of 'sleep raging', causing destruction in his sleep due to bottling his anger. After many 'anger management' lessons from Crikey, Monster was able to release his temper, stating he felt glorious afterwards. Same example was shown in The Dark Night when he tried to befriend a loose cyber monkey, despite its ongoing rampage and destroying his and Robot's apartment (followed by causing Robot intense suffering). However, Monster was able to win its friendship near the end of the episode.
Monster owns a pet (against the apartment rules) named Marf, who can only be described as a sentient block of steel with four small wheels for feet. Monster spoils his pet by giving lots of (undeserved) treats and coddling him. He's also known to dress Marf up in costumes (like a pirate's hat and eye patch). However, he often quickly hides Marf (in sometimes uncomfortable places) from Mr. Wheelie, the landlord, in order to protect the pet. Monster is very enthusiastic and will bear with bad news for as long as he can, and has the need to cheer people up, and cheer them on to keep them happy. Monster can be rather childish or dim, which is a contrast to Robot's more mature and cynical personality.
Despite his nice character and being kind to others, he is often upset or overwhelmed to the point where he becomes furious when things don't go as he hoped, such as throughout Boomerang when he had been desperate to prove karma exists. These emotions are also met in incredible rage sometimes towards Robot's own family. Monster's love, care and sympathy for his best friend and wishing he was on better terms with his mean-spirited relatives drove him to the point where he went out and confronted Arpa, Gart, and Grandma toward the end of Family Business, saying 'What is wrong with you people?!', despite having taught them the lessons of appreciating Robot even after he saved their family business from Pendulum Depot.
Nick.com's official description of Monster is as follows:
"Monster is the eternal optimist, living by the motto that "good things happen to good people," and that all people are good. An extremely outgoing Organic, Monster is endlessly fascinated by the little things in life. He's driven by the need to make everybody happy, and the need to explore the "shiny thing." But because of his child-like fascination with the world, Monster is easy to fool. Unlike his best friend Robot, Monster feels like everything always goes his way, even when it doesn't. This makes him a great guy to have around!"
Monster is tall with short purple fur, sporting two ivory horns that curve towards each other on his head. His body is rectangular, his head being the largest part of his body. His head/torso takes up a majority of his body, with the waist down being squat and stubby. His arms are on the lower portions and lack fingers (but unlike most characters in the show, he has thumbs). Monster wears a small, dark gray bowler hat and light blue pants with pockets. His eyes are black and beady, and his lips are a light lavender. Monster's tongue has been shown to be quite long. Monster's horns can bend depending on his mood-if he's sad, they turn upside down, and if he's excited, they might straighten up and then curl again. He's also trans-gender.
- Oddly, whenever his head is facing sideways or upside down his hat still stays on.
- It is revealed that Monster is an enthusiast in the sport of Pole-O.
- Monster's favorite food is bacon, the same as Robot.
- He is very good at Pole-O.
- In the episode Hornica, it is revealed that his horns will fall off if he gets too worried.
- He has a crush on JD, just like Robot.
- He likes to put on a wig and sing sometimes.
- It is revealed in the episode Hornica that Monster never went to college.
- He invented the head-scratcher in the episode "Monster Invention".
- A special facet in the series is Monster constantly taking out his wallet and showing pictures that either foreshadow the events of an episode or display past events that occurred in his (as well as Robot's) life.
- Sometimes when his eyes are directly in the light, it could be suspected they might be dark blue.
- His personality is very similar to Larry the Cucumber from Veggietales.
- He owns a 'security blankie' as shown in Apartment 3 1/2.
- Monster's friendship with Robot is similar to Sulley's friendship with Mike from the Disney/Pixar film Monsters, Inc.
- So far three members of Monster's family made an appearance but his mother has not appeared yet, however she was mentioned in Model Citizen.
- The use of his tongue is often a gag on the show (such as using it as a rope in The Package).
- He is the only main character (besides J.D.) that is Organic.
- He has the same voice as Apple from The High Fructose Adventures of Annoying Orange.
- The curve of his head resembles Gumby.