Aozora is Rikitarou's sister's child.
Spoiler, click to view
After Rikitarou's sister and some of her relatives pass away in a car acident after returning from a trip, Rikitarou raises him as his own child, as he is his closest relative(there were others but Rikitarou took it upon himself to take Aozora in after hearing that if he didn't, Aozora would end up at an orphanage)
