Joy Mutants are strange, deranged beings who were once normal humans, but mutated into body horror abominations due to their overdose of the drug known as Joy. Mutant transformations happen quickly enough that they have been known to surprise a group of people and massacre the rest.
The mutations seem to vary from person to person. However, there are some common traits among the mutants, such as elongation of the neck and arms, incredible swelling, and fusion of the head directly to the body. The mutated bodies are often asymmetrical. While some mutations can allow the person to maintain a vaguely human shape (such as Jonathan and Blue), some can be much more distorting and drastic (such as Bobby and Louie). Some mutations appear to make the person stronger or faster (such as Baby and Peter) while some make the person appear immobile or simply grotesque (such as Punkert and Arty). It is unknown if they are physically immobile or simply relaxing, but it is implied that, despite looking unable to move, they are still capable of seeking out others to kill (an example is Dice Mahone: despite looking like it cannot move, it is surrounded by corpses of people who one can assume it killed).
The psychological effect of the transformation is that the mutant completely acts on their deepest desires. Often times, this manifests as violence. Most mutants are surrounded by the bodies of those close to them, whether it is fellow gang members or close friends. In the rare cases where the thoughts of the human close to mutation is revealed, it becomes apparent that these desires can be sexual as well. When speaking to Blue in the third encounter with the Salvation Rangers in the desert, he says that he's having violent and intimate thoughts. Examining the Doctor's notes after killing it show that Dr. Lemont was having thoughts of not only seeing his wife again, but also feeling her, and ultimately killing her.
The men of Olathe often call them "beasts" and use "it/its" pronouns to describe them. The men of Buzzo's gang call the process of mutating "turning". The names given to Joy Mutants are often simply first names (Jonathan, Hawk, Louie, etc.) or nicknames (Blue, Beady, Baby, etc.), the exceptions being Henry Wyatt, Dice Mahone, and Rex Thunderstorm.
As noted before, most mutants are extremely aggressive. Often times, they are surrounded by corpses that are horribly mutilated, even cut completely in half. Since they often attempt to bite Brad, Buddy, Rando, and the other party members, it is implied that the mutants are cannibals.
Not all mutants are violent, however. Some mutants will only be violent for a short period of time or towards only specific people and then will not attack Brad or Buddy. This is seen on Resort Island, where a mutant is surrounded by the bodies of other scientists but will not attack Brad, as it is "done killing". Some mutants are satisfied and simply do not want to fight, such as the mutant in the Construction area who "doesn't look like it means any harm". Whether or not it is because they have reached their desires is unknown, but Brad and Buddy can simply just walk by them.
Most Joy Mutants can be controlled by Dr. Yado and his trumpet. He can play his trumpet in specific patterns to command them to do specific actions. His "personal" Mutant is Sweetheart, which is obedient to him. He can even command it to kill only specific people, as seen when he orders it to kill Bolo specifically and not Buddy, despite them being in the same area. He often will play the same tune throughout all of Olathe, apparently controlling them from afar. He notes, however, to Buzzo that he cannot control every Mutant, as some are not responsive to his trumpet. The trumpet is what can control most Mutants as opposed to Yado himself.
Brad and Buddy will most often encounter these enemies when they are blocking the way. For example, Charmy blocks the way to the Child's Bicycle and Harry blocks the way to Green's House. Still, some will actively seek out Brad or Buddy, such as Peter leaping down from a cliff to attack Brad and Cooper slowly walking towards Buddy once it sees her enter the area.
Joy mutants show similar trends in battle. Most mutants have low speed but high HP, meaning that they will take a long time to kill. They also have several status or self-damaging moves that they will often use. Their movepools will often reflect the traits on the mutants. They will often glare at or attempt to lick Brad, Buddy, and their parties, reflecting their intimate or sexual urges. They also resort to attacks like bites and scratches as opposed to formulated punches and kicks, showing a deterioration of the mind (however, some still know how to break a neck). Also, they often cry, scream, and cough up blood in battle as opposed to targeting the party, indicating that they are in pain or simply get distracted by their own form. They are notorious for being able to permanently kill party members. This is often done by completely biting one's head off. Not all mutants will frequently use these perma-kill moves, and some lack the ability to execute these moves altogether. On the other hand, some will often kill Brad's party members due to their high frequency of using a perma-kill attack.
In LISA: the Painful, the non-playable men of Olathe are most often unaware of their presence. Many of the men take Joy often, most likely not knowing of the fact that they may mutate as a result until it is too late. They often do not interact with the mutants directly in their area, whether the mutants are peaceful or not. For example, if you speak to Nern after Punkert has mutated, he will not acknowledge it, despite it being onscreen at the time. The exception is the man who sits in front of Charmy in the Playground. He remarks that he has watched it kill dozens, and yet he sits and watches it smile. He also refers to it as "poor thing" when Brad kills it. Remarkably, it appears that Charmy has not attempted to kill the man who stares at it.
However, in LISA: the Joyful, as more mutants start appearing more frequently, the other men of Olathe take notice. Early in Buddy's journey, she encounters Lenny Luigal and his gang after they have successfully killed a Joy Mutant, despite it being a difficult battle for them. An unsuccessful battle with a Mutant is seen in Vega Land's Cave, where Bobo is seen surrounded by bodies but also is covered in scratches and has arrows and knifes lodged in its body. The men of Sindy Gallows' gang form a cult around the Mutants, and they betray their leader after he makes attempts to kill the Mutants of the area. Regardless, more and more Mutants appear across Eastern Olathe as Buddy's adventure progresses.
Be careful of these enemies. Their appearance can be misleading and while they spend many turns using moves that don't necessarily cause physical damage to your party (glare, crying, coughing up blood, screaming, etc), they also have high damaging or even potentially fatal moves that will permanently kill your party members.
Lisa: The Joyful Edit