Short answers, pulled from the story.
Kurau is a twelve-year-old girl whose body was taken over by two sentient life forms called Rynax, while Christmas is the second Rynax that emerged ten years later as a twelve-year-old girl. They exist as sisters despite their different origins, with Kurau possessing superhuman abilities like flying through solid objects and Christmas developing emotional bonds and quick learning skills.
The series aired on Animax and TV Asahi networks between June and December 2004 during a six-month broadcast window. A.D. Vision licensed the show for North American localization starting from when the first episode aired and released the initial DVD set on April tenth two thousand seven before transferring rights to Funimation Entertainment in two thousand eight.
A human merged with a Rynax becomes known as a Ryna sapien, though this outcome usually leads to slow agonizing death for both parties involved. Kurau proves to be extremely fortunate since the human and Rynax components live together in symbiotic relationship without destroying each other unlike most other fusions which result in nearly immediate death.
The Global Police Organization pursues Kurau and Christmas under belief that they represent threat to world safety while Commissioner Saito commands GPO unit tasked with investigating and controlling Ryna sapiens. He intends building military unit out of Rynax soldiers regardless of human casualties involved even though Agent Doug begins empathizing with Kurau as she goes on run through city streets.
Kevin Gifford praised first volume for neat take on super-powered ladies formula saying it had one of most memorable first episodes among recent anime releases while Diane Tiu gave series five out of five stars claiming very little to dislike since no filler episodes existed. Theron Martin named Kurau and Christmas Best Couple describing their close loving connection as putting even Keiichi and Belldandy to shame and IGN reviewer Jeffrey Harris gave first DVD six out of ten rating stating show starts slow but could pick up steam if less vague about origins.