Yep! Related to them!
Lies about stuff that doesn't even matter, lied on cv's then shocked when they can't do the job and get sacked, lied about health, relationships, crimes (being witness to - not officially but certain "famous" crimes claimed to know someone attached to the case and be "in the know")...
I do think it's genuinely a mental illness issue but also that they are fully aware it's all fantasy.
They hate not being centre of attention and throw a strop if caught out.
They are no longer in my life and so I don't have to deal with it. I'm still learning some of it though through mutual acquaintances. Now that they know that I'm no longer in contact they seem to feel safe telling me stuff, and it's FAR worse than I previously thought, barely the right side of legal - just barely! Though I have my suspicions about potential fraud, I can't prove it though.
I don't think they're particularly rare, but again that might be personal experience, as I also have a branch of the family that are actual con artists but that has a clear goal I guess so isn't quite in the same area.
What I do think is the person they're deluding the most? Themselves.