He is an elderly Caucasian male, and seemed somewhat thinner and taller than his other descendants.
His soul was summoned by The Grim Reaper to prove Marvin Marsh his suicide wish was not the right thing to do. According to his soul, he was stuck "in limbo" for all these decades ever since his grandson, Marvin (who just turned 102), killed him when he was a child, never making it to heaven nor hell, because he skipped the natural passing away.
The name confusion Marvin has appears to be genetic, as the ghost of Stan's great-great-grandfather called his grandson "Billy" instead of "Marvin". His soul was taken back by Death alongside Kenny's soul who was just killed by Death's touch.