Background: Kira is an interesting character -- he's had many "backgrounds" in each in his incarnations, but his first was that of Lucifer/Lucifel . However, in the body of Sakuya Kira, he's existed probably for the longest out of any of his bodies. Kira has been a wandering soul for a long time, taking over the bodies of others - in this case, a young boy - in order to find Alexiel.
Basic: Setsuna's best friend - his true incarnation is the lord of hell, and the blood scar across his chest is Rociel's mark - where Rociel's blood fell upon him and made him an immortal soul, able to survive any wound inflicted upon his body.
Information: Sakuya Kira is actually the spirit of the Nanatsusaya - Lucifer , the lord of hell, imprisoned in a sword by the Angels. The real Sakuya Kira - that is, the body that Lucifer is using - is that of a boy who died in a car crash with his mother. (To differentiate between the two, Sakuya will refer to the child, Kira to the character that is Lucifer) Sakuya asked that Kira make his father hate him, so that when the time came for his death, his father wouldn't be sad, and he also asked that Kira read a lot of books, because Sakuya had always wanted to do that. Kira acts cold and uncaring, but beneath the cold exterior there really is a heart, and through using human bodies he has begun to learn human emotions. Kira loves Alexiel and at the same time hates her for it, and has followed her through all her lives, protecting and watching over her. The blood mark came from when Rociel's blood splashed across Kira's chest, but the blood is weakening, and so recently Kira has had a hard time regenerating. What can kill Kira even with the blood mark is if the nanatsusaya is damaged - if the pendant cracks, Kira bleeds. Alexiel tells Kira that one day he will return to being the Lord of Hell, and he will stand in Setsuna's way when he tries to win the war against Heaven. Kira is killed by Rociel as he's protecting Setsuna , and the nanatsusaya shatters. However, creatures that seem to be part of the nanatsusaya follow Setsuna , still trying to protect him, releasing Setsuna from his cage in Heaven when he's imprisoned. Kira is then revived by Rociel , who believes he controls Kira because of the blood he carries on him. However, Katan believes that Kira is very much still aware, and is only doing what Rociel likes for his own reasons. Katan is correct. Kira had slashed at Katou and had had the chance to kill him once - and yet held back. This was his human side shining through the evil. Sakuya, the human in Kira, is a part of Kira's consciousness, and was within the shard of the crystal that Setsuna had within his bracelet that he used to call Nanatsusaya (after Kira's death and resurrection as Lucifer). Kira is one of the main players, however, when the earth is saved, and in his moment of humanity where Lucifer is gone, Kira tells Setsuna to destroy the Heaven's Tablet computer which will stop the earth's destruction, as a fullfillment of his promise to Alexiel. Sakuya finally, when the world is unfrozen, gives his father a hug, showing him that he really did love him.
(For more information, read Lucifer's page in the Hell section)