The poster character from Beatmania IIDX 12th style: Happy Sky.
She is actually Nyah's long-lost younger sister, who was believed to have been killed in an accident. The trauma of the accident left her with amnesia, and she spent several years being cared for by a classical piano player who traveled the world (giving Xia the opportunity to develop her characteristic tan).
She now lives with her sister and attends the same school as Daruma and Hishimiya Tsugaru.