This is one of the two demons a demonologist may have. A male demonologist will have a succubus, while the female demonologist gains an Incubus instead, unless they use a skill beforehand. They are equivelant, aside from the skins.
At the moment, all 5 demons appear to summon a succubus or incubus. This may change later; a Demon Servant and a Demon Warlord are hardly expected to look the same.