The following are skins that cannot be selected at the pause screen, but are part of the game. Press ~ to display the console window, then type \model . The skin names are as follows:

reborn/boss (a reborn with a red/dark blue color scheme)
reborn/acrobat (reborn with a blue and black color scheme)
reborn/forceuser (the reborn in the picture at the pause screen)
reborn/fencer (reborn with a green color scheme).