I really related to Kazuha and I ended up crying when he decided to disappear. Seriously, I miss every single alter so much, they were all beautiful. I loved this story, I'm glad I read it!
I think he had a twin brother who was a lot closer to nabi than him, and then out of jealousy he probably tempted to murder his brother but was unsuccessful. And now his parents live with the brother to take care of him (who is probably disabled in some way now).
I'm late but happy birthday!! I hope you enjoyed your day ^^