This Rukia is wearing traditional clothing from the island of Puerto Rico, known as Borinquen by natives. Its tradition is a bit more diverse than some countries (although it's technically a territory) because it's comprised of three cultures brought together: the Spaniards, the Africans from the slave trade that passed through, and the Taino indians who inhabited the island before the Spaniards came.
Rukia's dress is based mostly off the style worn for Spanish style dances, although I added a tribute to the African bomba in her headdress. Okay, well, enough of my rambling.
If someone could pass this along to Koto, I'd be so grateful! :3