*MAJOR Spoiler WARNINGS*
Sakuya Kira is probably my favorite character. In the image to the left, he's shown with Setsuna. He is Setsuna's best friend and Setsuna looks up to him as a big brother. He's 18 years old.
When Kira was around the age of seven, he was in a car accident. Everyone else, including Kira's mother, was killed. As Kira lay dying, all he could think was that he didn't want to die. He was approached by a Samurai who made him an offer: if Kira would allow the man to fuse with his soul and live in the body of Kira. In exchange, he would do all the things Kira wanted to do before he died (mostly read books).
Kira's last concern was for his father. His father would grieve horribly for his mother and Kira knew his own death--after the Samurai was finished with his goals--would bring his father even more pain. Kira made him promise he would make his father hate him so that he would not mourn over his death, but Kira's father loves him so much, that turns out to be impossible.
The Samurai is really the current incarnation of Lucifer, the angel that fell from Heaven. Because he is fused with Kira, he feels human emotions. He loves Setsuna like a brother and protects him all the time. He is the only one who doesn't condemn Setsuna for loving Sara.
Kira is cool, not only because he's part Lucifer, but because of his love for Setsuna. He deeply cares about him and accepts him for who he is. He does not judge him. Everyone needs a friend like that. He's even willing to die for him (and does). Kira is also my favorite because of his struggle with emotions. He can't quite understand why he loves his father or why he needs to protect Setsuna, but he does.
For an awesome Lucifer/Kira Shrine (and much more detailed info on one of the coolest AS characters), visit Jigoku.