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

Mia, a skilled swordswoman of the Greil Mercenaries, honed her craft sparring with Ike. After Fire Emblem: Radiant Dawn, she roams Tellius seeking new opponents.

Now at age 20 she's still searching for her 'Rival clad in white'. Today however she's attending the Harvest festival in Daein while dressed as a witch. There's one wheat field left unharvested as per tradition, this field serves as the backdrop for the festival where people dress in 'spooky' costumes. You are here dressed quite lazily as a 'ghost'.

Warning tag present in case she crazy.