I imagine a skelegirl as a kind of ghost: a transparent, semi-tangible ectoplasmic ghost body around a skeleton, skelegirls are formed as an intermediary state between a zombie, an intelligent undead reanimated corpse, and a full-fledged ghost, when she is first ressurected, her body is reanimated via ectoplasm, and she's a zombie, as she decays, she becomes a skele, finally, her bones fall away, and she becomes a ghost.
this is the metamorphic cycle of undead girls, not mentioned is the first part, as that is just a living girl, also, zombies retain all memories and personality from being alive, as well as all other steps also retaining the information from the previous ones.