Kirainein or Kira as she is called is a dark goddess from Greek mythology known as a "Keres" .....She is a daughter of Erebus & Nyx (Greek Myth) ....She takes on the human form (with elf ears) of an 8-10 year old child and while her species are thousands of years old she is a youngling. She is also known in American mythology as a Grim Reaper and devours souls to send to Hell/Hades. She is quite evil but very cute and can be sweet when she wants to be. 

Kira had been travelling across the world and came to rest in a forest where she was chased by creatures she had never encountered before. She tried to hide among the trees but fell and injured herself. She was then found by a young female named Yuiko. Yui took her in and took care of her and treats her as if she were a little sister. She owes Yuiko her life and has vowed to live with her until the end of days. Lucky Yui, eh? 

Sadodere