That’s Maria from Knights in the Nightmare, well, this year’s Atlus-O-Weenie contest winner dressed up as her. The impressive outfit was handmade out of polystyrene and celtic (armor), linen (skirt), and wood plus insulation board (sword). Atlus readers selected this costume as the best of the bunch.
There were other fantastic submissions like these Shin Megami Tensei: Persona 3 and Persona 4 costumes of actual personas. You can see all of the winning entries (including a quirky Touch Detective one) here.
Correction: Atlus readers picked Maria is the fan favorite costume. The staff at Atlus selected Thage from Eternal Poisonas their top pick.