Yamato is someone who thinks of others more than anyone else, protects what should be protected, and cares for others despite having his own problems to deal with.
It is later revealed that his necessity to help and protect others comes from his middle school years when he couldn't help a friend who was being bullied. He has been feeling guilty all these years since the reason he didn't help his friend was for fear of getting bullied himself.
Yamato can also get extremely jealous when someone shows an interest in Mei, as seen with Kai, when he punched him. Yamato is also somewhat impulsive, doing things without thinking about them.
When Yamato was in middle school his friend was a target of bullies. Instead of helping him, he tried to avoid him in public at all times, in fear of being targeted by the bullies. However, he hanged out with him secretly. One day his friend thanked Yamato for hanging out with him and the next day he transferred schools. Yamato has felt guilty since then.