This is supposed to be Illya from Fate Prisma Illya, but I can't find a visual clue that definitely makes this Illya. If only there was even one shot of her wand in the background or the unique clock in her room or something. Otherwise, this set runs the risk of being a "generic cosplay of a catgirl".
*UPDATED* Yes, this is definitely Illya because her leotard design with the whitish center and varied black edging is from the Fate Kaleid Liner Prisma Illya show.