Mister Frees was a man of unknown species who is now a ghost.
According to a story once told by the Ghost Host, a book by Frees was among the volumes in the bookcase in architects Mr Davis and Mr Coats's study as they got to work designing Gracey Manor. Whoever he was, “Mister Frees” received a burial in the family graveyard of the Mansion, with his epitaph claiming that his voice would forever “carry on the breeze”.
Behind the scenes
Mister Frees's name first appeared in the 2005 story Blueprint for Murder before being given his own tombstone in 2011 in Walt Disney World's Haunted Mansion. Like all the names of authors in Blueprint for Murder (and like most tombstones in the Haunted Mansion), the name is an allusion to one of the original creators of The Haunted Mansion in the real world — in this case, Paul Frees himself, who voiced the disembodied Ghost Host.
The epitaph interestingly hints at “Mister Frees” becoming a disembodied, ghostly voice in-universe, comparable to the immortality of Paul Frees' performance as the Ghost Host. This may even be taken as a hint that “Mister Frees” was in fact the mortal identity of the Ghost Host.