Shoya is the first secondary character in the game, and one of only two who are not originally part of the Black Swarm. He is characterized by his overprotective tendencies for Yui, and his cool disconcern for everyone but her.
Like most characters in the game, however, he is more than simply that. He is highly intelligent, and is as honest and caring as the rest of the kids, but seems to have confused feelings about his self-esteem and worth in the world. He tries to act cool, and outrightly states in the beginning that he doesn't care about the dying Cimexus, despite the fact that everyone around him shows their emotions and would like him much more if he did too, and he's smart enough to know this. It could be that he sees how pure Yui and Yamato are, and doubts his ability to emulate them, so he acts out and cements his identity as heartless, but impressive nonetheless, and more importantly, a protector from the outside- moreso to himself than anyone else.
It's hard to believe his arguments between Yamato and himself aren't intentional. He always starts the argument by saying something mean, which will obviously make simple Yamato say the good and human thing everyone in the room is saying. Despite the fact that Yamato never says anything unreasonable or wrong, everyone always blames both of them. Because he has cemented his identity as heartless, this does no damage to him, but he gets to drag Yamato down to his level for that moment, getting a laugh and an ego boost.
Shoya and his drone A-Geeto join as support mid-Chapter 3 and join the team afterward.
"A-Geeto" is a dual-reference to "Agito", which when read in japanese, means "jaw", often a dragon's- but when read in latin, means "move".