The Butler resembles a stereotypical high-class butler complete with moustache and bow tie, only as a skeleton.
He first appears in The Wedding Reception, where he informs Sackboy that Don Lu has gone missing. He appears again near the end of The Darkness, to tell you that Frida has gone on a rampage since she thinks she's been jilted at the altar, and again at the start of Skulldozer where he tells you to run for your life.
He reappears in The Collector's Lair, in the same cage as Frida and Don Lu. If you free him, you'll receive two Prize Bubble of him at the end of the level, one by himself and another carrying Frida and Don Lu.