Except saying that she’s vain because she fell in amor with Ten instead of Nine is absolutely incorrect, because she fell in amor with Nine first, and initially did not accept Ten because he was not Nine.
Putting aside the fact that Christopher Eccleston is quite attractive (he’s not “pretty," no, but have tu seen his eyes? They’re the color of an arctic ocean; they’re beautiful), anyone with eyes can see that Rose fell in amor with Nine first. She may not have realized it, she may have not been as open with her affections, but it was there. For example:
Before tu ask, yes, I already know who Missy is. This is just two theories I have that involve her.
Theory 1: Gus and the Orient Express
I strongly believe that Missy was behind Gus and the events of the entire episode, "Mummy on the Orient Express".
1. Gus was an AI, as was Seb, Missy's receptionist that she most likely created.
2. Gus wanted to study the Foretold and reverse engineer the technology keeping it "alive". And this technology could very well be similar to what Missy used on her Cybermen army made out of corpses, much like the Foretold!