An amorphous creature capable of assuming the form of any person, most frequently a smiling nun, who aids the Skullgirl.
A 'mimic' character, each of her normals looks like that of another character but will have vastly different frame data and properties. Though she transforms into other characters during her specials, they are completely unique.
Double is voiced by Charlotte Ann. A DLC voice pack will include alternate "Republican Double" vocals (an inside joke amongst the voice actors) provided by Kaiji Tang.
Double's story begins with her offering the Skull Heart to Marie, enticing her to become the new Skullgirl, she is then seen talking to some unknown masters stating that she will begin collecting candidates (in other words the other seven characters) immediately. Bloody Marie sends Double to destroy Lab 8, but tells her to leave Peacock alone saying she made a promise, Double states that promises are irrelevant, that they both exist only to serve their masters. In the next conversation with said masters, Double mentions that everything is proceeding rapidly, but that Marie is somehow wielding the power of the Skull Heart to further her own agenda.
After capturing the seven candidates, Double mentions to her masters that harvesting is behind schedule and receives orders to eliminate and replace Marie. In the catacombs below the cathedral, Marie states that as soon as the Medici Mafia is destroyed, she will comply with their master's wishes, Double merely replies that as she denies her purpose, so too do they deny her continued existence and attacks Marie. After defeating the Skullgirl, Double states that Marie's spirit was too powerful to allow the Skull Heart to fully penetrate it, thus synchronisation was denied. She goes on to say that she has chosen a new host, and that the remaining candidates will be consumed. The next frame shows all seven of the remaining Skullgirls roster being absorbed by Double.
The camera then pulls out and reveals that two other characters, Aeon and Venus, have been playing Skullgirls as a way to trace the threads of fate for each of the characters. They mention that each of the characters storyline arcs is a thread of fate that might have been, but that one true thread exists which will unite them all, including Aeon and Venus' fates. A third character, called Mother, appears and tells them that their dinner is ready.
The three characters at the end of the story are seen on the stained glass in the cathedral, indicating that they are the three goddesses of the Divine Trinity - the fictional religion in the world of Skullgirls and the one with which Double's nun is associated.