A kind of interesting phenomenon. He comes in with his dog, cries that he doesn’t have a place to stay, Jon allows it for as long as needed and then… he just vanishes one day, leaving Odie with Jon, never to contact them again. Did something happen between the two? Was he ever real or a product of Jon’s mind? A wiki states:

According to Davis, Lyman’s original purpose was to be someone who Jon could actually talk to and express other ideas — a role gradually taken over by Garfield, himself.

It doesn’t reveal who gave Lyman that purpose; it could be that it was Jon himself who, over the years, got less attached to reality, so he got done with talking to and interacting with Garfield.

That or it’s just a lazy uninspired comic that only has a minimum level of continuity and doesn’t care to explain why a former choir character suddenly vanishes.