Halua is the older sister to Emil and both were once part of a human experimentation to create the ‘ultimate weapon’. Halua, No. 6 became the ‘ultimate weapon’ but retaining her human heart resisted and went berserk. As a way to contain her the researchers used Emil, No. 7 to keep her at bay.
Emil accidentally merged with his sister Halua gaining her powers and a body reminiscent of hers.