Ken is a ghost who used to be a man of an unknown species.
“Ken” was apparently a friend of the Gracey family. He was buried outside Gracey Manor, with his epitaph reading “Drink a toast to our friend Ken, fill up your glass and don't say when”.
Behind the scenes
Ken is a tribute to foundational Haunted Mansion Imagineer Ken Anderson. His tombstone was one of the new elements of the 2011 expanded queue to the Walt Disney World version of the ride. Unrelated to this tribute, Blueprint for Murder, a 2005 comic story, had featured a nod to an architect simply known as Anderson. It is unclear whether the two are one and the same in-universe Ken Anderson.