The Forgotton Man represents Kris's biological parents.
Kris never met them, so the man does not visibly appear, but they see apparitions of them everywhere- in a car, in the diner, etc. Kris is obsessed with the idea, and that is how the man manifests.
In chapter 3, the man appears in a world of lost items, that ramble on about a lost childhood and how much Kris has "grown up". In chapter 4, the man appears in a hospital (possibly where Kris was born?), hidden in the scene transition to a room with a panel saying "Roots"