Schizotheory incoming
Here's how FRIEND could be Dess and Asriel's dead child without the story suddenly getting really dark.
Cats are sometimes meme'd as "child replacements" and given that Cat Petterz exists, we can assume that the concept of pet cats exist in this universe(like a Goofy vs Pluto situation don't think too much about it).
So Dess and Asriel find a stray cat one day and adopt her. They're both teens/tweens but really want to grow up and be adults so they treat the cat like a daughter and LARP as parents. But one day Kris takes the cat out for a walk and she dies. Maybe Kris was directly responsible, like they threw the cat into the lake or incoming traffic during an autistic episode and they feel terrible about it.
So Kris doesn't immediately wanna confess that they got Dess and Asriel's child replacement killed, so they grab the cat's dust and confide with Catti and Jockington who agree to help Kris to do an occult ritual to revive the cat.
But Jockinton actually has ulterior motives of sorts and isn't actually here to revive the cat. He secretly sabotages the ritual to insert his SOUL into the cat corpse. So the cat turns into this fucked up demonic creature known as FRIEND.
Kris bags FRIEND and tries to dispose of it maybe flushing it down the toilet, so somewhere within the drainage system. But this was all part of Jockinton's plan to take control over Hometown. Ridden with fear that the thing will crawl out of the pipes, Kris ritualistically flushes ICE-E bath bombs down the toilet and they obsessively wash their hands in case any of FRIEND was left behind.
Eventually, Dess and Asriel find out that their cat died and they make a plushie, Seam in memorial. Things are normal again until the first Dark World was created in the shelter. A voice starts crying out to her "mama" and Dess hears, she thinks it's her deceased cat so she follows it, only to get absorbed into the shelter Dark World and her SOUL gets corrupted
