Oh this poor guy...
After his "loving" Master died, Mort left the lab he was held in and searched endlessly for a new one until he found Chet. The two had accidentally bumped into each other and when Mort saw Chet, he asked him to be his new Master. When Chet said no, Mort just chose to follow him like a lost puppy. Chet tried to tell Mort to leave, but Mort took this as his new Master being unhappy and so he, w/o hesitating, changing his expression, or even flinching, raised his hand and punched right through his chest (there hemolymph pouring out and everything). Chet, upon seeing this, realized that Mort wouldn't leave him alone until he accepted and that Mort was seriously messed up in the head so he finally caved which caused Mort to get down on all fours and make a little squeal. From this point on, Mort would go everywhere w/ Chet, sometimes it'd be helpful like when Chet got a new job, or it could be really awkward like when Chet had to visit his mom.
Mort mainly attacks w/ physical blows and bites and while he has wings, he doesn't really use them for flying as he's a pretty clumsy flier, but instead he uses them for evasive maneuvers. He won't do anything w/o Chet's consent and that thing around his neck is his collar (those, except for Chet, who try to remove it will be attack w/o mercy).
Megalovania: Mirrored Insanity and/or Dustshift The True Demon.