Unfortunately, it's pretty much canon that Exile is female, so if anyone ever writes a tie-in novel, it's going to contain that horrendous nightmare of a love interest. Fortunately, we can probably dodge it in most other mediums.
Nihilus had no character. His whole thing was just Kreia, except with a Vader Mask and less interesting. Much less interesting.
No, she doesn't carbon copy anyone from Star Wars, because Star Wars usually takes the Megalomaniac approach instead of the Nihilist approach.