Name: Yoite. It's the name Yukimi gave him when he was first sent to live with him. Yoite sort of unintentionally reveals to Miharu that his original name is Sora (and that he does not consider himself to be that person).
Appearance: Yoite's appearance isn't particularly noteworthy - he's a pretty typical anime teenager (skinny, pale, effeminate face, etc.). The only really interesting thing is learned in a flashback of Yukimi's. Due to learning kira, Yoite's growth accelerated. Over the course of three days, he grew from a little kid to a the way he looks as a teenager in the present.
Age/Birthday: 16/the latter third of December. His age is a bit sad, honestly, because he's almost painfully young to be the powerful weapon of a ninja village. It's no wonder he has excess emotional issues - he's really young.
Family: He doesn't seem to have one. He lives with Yukimi, a fellow ninja, and he and Miharu latch onto each other, but for all intents and purposes he's alone in the world. It's revealed that he was an abused child, abandoned by his parents. The closest thing he has to family is Kairoushuu and Miharu.
Special Skills: Mastery of the forbidden art Kira.