Grim Reaper is simply a skeleton in a greyish-like robe (which makes it face or skull; completely unseen) The Grim Reaper only made an appearance in the 1929 theatrical Mickey Mouse cartoon, "Haunted House". It later became forgotten or an retired character after that cartoon and was sent to Cartoon Wasteland.
In Epic Mickey
The Grim Reaper reappears residing in the Lonesome Manor and is seen in one of its Projector Screens. It is seen in the first Haunted House scene before exiting, on the stairs, a shadow of the Grim Reaper will come. After the shadow reaches the end, the Grim Reaper comes out from the door and can walk down stairs and repeats again. Touching it can damage Mickey. The Grim Reaper is also mentioned in the Manor Graveyard on one of the tombstones that are stable. In the Dark Beauty Castle when you are going to the Sorrow Tower from the Fireworks Control Tower, there is a little statue of him.