Double is one of the playable characters in Skullgirls. Her homestage is the Grand Cathedral, but Gehenna is also referred to her "domain" and is even suggested to be an extension of herself.
Despite MikeZ stating that Double was female, her gender is still listed as Unknown on the official site.
The nun form that she takes is based on no one in particular; it's just a form she takes to blend in at the cathedral.
Double is speculated to have been created by the Trinity as she shares a few physical traits from each member.
Venus views Double as a maid or a butler, although it’s not quite as straightforward as Double represents many things.
Double (JP: ダブル) is a monstrous, transforming creature working under the Trinity, responsible for ensuring that the Skull Heart falls into the "right" hands and manipulating events according to the goddesses' will. Her current guise is as a nun under the name Sister Agatha.