Xing ( 心 , heart) {right} and Ai ( 爱 , love) {left} as they appear as a result of Nii's experiments. The twins are what Nii calls Artificial Halfbreeds:born human, but infused with youkai flesh to make them not truly either species. He also accelerated their growth through the use of both youkai magic and scientific means (such as growth hormones) so that they have the physical and mental appearance of sixteen-year-olds rather than the five-year-olds they SHOULD be. Whil Xing has assimilated fully and flawlessly with the youkai tissues, Ai has not. Her body has not completely rejected them, but it does recognize them as not belonging, sometimes causing sudden attacks of pain and weakness, which can lead to black outs. During her black outs, Ai sometimes recovers vague memories of the time before they were taken by Nii (a time which neither of them remembers clearly, including their mother, either due to how young they were or by yet more of Nii's experiments). She has vaguely recollected a lullaby Yue sang to them, and will often hum it to herself, or sing it to her sister when she needs to be calmed. Between the twins, Xing is the tougher in both physical sense and personality. She is a strong fighter, able to take out an opponent with her bare hands or a range of other weapons. She is serious toward her mission of attaining the scripture, and is very cold to individuals other than her sister, whom she is very protective of and who is the only person to see her rare sweet side. Ai is more agile than her sister and has better Chi control, fighting in a style similar to Hakkai's. She is warmhearted and soft-spoken, not fully opening up to anyone but Xing. She adores children and small animals, once even halting a fight she was in because she did not want to injure a cat in the crossfire. She often questions their mission for the scripture, and expresses a dislike for the assignment, but follows orders for fear of what may happen to her and her sister if she were to refuse.