Currently, little is known about about Monster's past.
From what is 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 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 has teased him similarly to how 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 bucks.
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. Monster also thinks there is good in everyone, even Robot's older brother,Gart, and tries to be nice to all.
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 personality.
Monster is tall with short purple fur, sporting two ivory horns that curve towards each other on his head. His body is rectangular, his 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. However 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.
- 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 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 with Sulley's friendship with Mike from the Disney/Pixar film Monsters, Inc.
- So far three members of Monster's family made appearance but his mother has not appeared yet, however she was mentioned in Model Citizen.