Sheep is one of the many other programs in The Talos Principle. Though he never appears in game, he's left several notes behind in the form of QR codes.
Sheep is one of the more independent and questioning programs, distrusting Elohim, and annoyed by Milton. He seems determined to find some sort of answers or purpose to the world he's in.
Sheep's name is likely ironic in nature, given that neither Elohim or Milton seems to have much sway over him. It also ties in with The Shepherd who knows the answers that Sheep is looking for.