Chaz is a Joy Addict turned Joy Mutant encountered in House Dust. You will have the option to give him Joy when you first meet him, but he will turn into a Joy Mutant after you rest, regardless of whether you gave him Joy or not. After turning, he will massacre the denizens of the inn, including the bartender.
Chaz is the only enemy in the game that can inflict Brain Damage, a status condition that can only be treated with the Trauma Kit.
You can completely avoid fighting Chaz by escaping or simply leaving House Dust, leaving however will cause the message "No use." to appear when trying to to re-enter the building.
Killing Chaz will grant you access to a record player, where you will find an Eerie Record. Trying to obtain said record before Chaz mutates will result in the barkeeper yelling at you to put it back. So far the record has no use in-game.
The two attacks that Chaz can do physical damage to party members with, Bite and Scratch, can only be used on Turn 2 of the encounter, according to the game's files. This may be a bug, but as a result, Chaz's only damaging attack beyond this turn is Devastating Chomp.
Chaz is the enemy with the highest HP in the entire chapter, the second being Roland Von Buckingham with 2,500 HP.