This is why I find it so stupid she can't marry Chrom. I think it was game design choice so that he wouldn't be able to marry two pegasus knights but he is in so much of her dialogue. At least with Sumia (besides cutscenes) the only thing really is her item finding dialogue and that can be interpreted as a sort of little sister relationship and wanting to show big brother the cool thing see found.

She joins before Chrom is forced to marry. She shows up during the war with Peligia.