Wow. She's incredible. I like the designs on her clothing. It really evokes that sense of "this is someone who has an affinity for fire and quite possibly burning things with such". I also like her girl-abs. <3
It isn't exactly fire. Basically, in this scene, she experiences the full effect of the blessing she received from Asuna, the Planar-Lady (Goddess) of the Moon and Stars. Her affinity, in this state, is more towards light than fire, but combined, it can burn through any darkness-aligned creature, including skeletal undead.