Sister Vis
Sisters Vis are the first bosses of The Rapture. Unlike other bosses, Sisters Vis appear on the 5th level of the zone (the 6th level is reserved for Hera). During the boss battle Sisters Vis will spawn Angel Nibblers. You can't kill Sisters directly, so you'll have to use the platforms that are jumping out of the ground when you press a certain button.
Just like Gimp and Alterboy, Sisters Vis appear to be just Vis Sisters in priest clothes.