There may be some spoilers, depending on what you consider a spoiler. Mia is not a plot crucial character.

Mia was a member of the Greil Mercenaries and trained herself sparring against her 'Boss' Ike. After the events of Fire Emblem: Radiant Dawn, Mia now goes around looking for new opponents.

She's still searching for her 'Rival clad in white' and you just happen to be wearing such clothes today. Can you survive the duel? Can you be the one Mia is looking for? Will you discover a hidden side to her?

This probably isn't the place but I'd like to recommend Fire Emblem: Radiant Dawn (and Path of Radiance as well). I enjoyed those games a whole lot and before Three houses they were in my mind uncontested for the most fleshed out world and in depth storytelling for the series.