"—Lorath, have you spoken to Orek yet?— Tyrael and Lorath Nahr(src)
—What does one say to an ancient nephalem ghost?
—I suppose you start with "hello"."
Orek is an NPC in every town during Adventure Mode of Diablo III, standing right next to the Nephalem Obelisk. He is an ancient Nephalem spirit, and the warden of Nephalem Rifts. He can be spoken to (but will not tell much). Upon entering the Rift, he will also wish the player luck, and will warn when the Rift Guardian spawns.
Overall, the only real interaction with him is speaking after killing a Rift Guardian to turn in a mini-quest. This will begin the closing sequence of the associated rift, and will award all players with gold and experience, as well as summarize the results of the Greater Rift run.
Urshi will mention him in her conversations as well.