For one, she's got two chapters on him.
Two, you can have her being a tier three unit going into part three because of how much BEXP you get from 2-3. You can't have Aran as a Tier three unit without screwing over every other character in part one. (If even that.)
Three, who else are you going to use in part two? You pretty much have the Knights, who have 3 1/2 chapters to play in until endgame, Lucia one chapter before endgame. You have the laguz, and Marcia. Harr and Elinca are only there for 2-P and 2-F. And Brom is ugly. Heather's not to bad though, she does net you a bolting if you trained her.
In part one, I have Miccy, Sothe, Edward, Nolan, (I like to train Ilyana as well) Jill, Volug, then you get the BK and Naliah. Why would you ever want to use a solider that some people kill because they thought he was just some random?
That's three points in Neph's favor, and that doesn't even take stats into account.