Complex sentence: At first, Nate is disappointed that he didn't receive a brother, but when he goes to feed Diamond Destiny, she flings the bottle across the street using her ninja skills, therefore changing Nate's mind.
Split into simpler sentences: At first, Nate is disappointed that he didn't receive a brother. But when he goes to feed Diamond Destiny, she flings the bottle across the street using her ninja skills, therefore changing Nate's mind.