I like
Christian for a girl but I would never use it since I love it for a boy.
It's kind of frustrating for me that so many think that
Christian could only be used if you are
Christian or want to raise the children in a religious home. To me
Christian is first and foremost a name. I mean, just because you name your child
Parker or
Hunter doesn't mean you have to be a parker etc (OK, maybe a bad comparison but still...)
Anyway I think
Christian would be nice for a girl. I like
Christian Lily as a combo because
Lily is so feminine.
You could use
Issy as a NN.