While he was away as a kid, Black / Yggdrasil went to his city and killed his entire family. With nowhere else to turn, he decided to joined the military. It quickly became apparent that he was no ordinary kid, as he uses the shadows to his advantage and had always swore to kill Black.
After Jenna Liddell sealed Black / Yggdrasil away, Kainn's reasons for staying in the military have become less clear. Although hes starting to see the coruption he still wants to believe that the government can change.