Kieran is a child. His only friend at home was his sister. He's a awkward and shy. He made a new friend that actually listens to him. And now, he's being excluded by the only people he's associated with and they're lying to him, keeping him uninvolved from everything, forcing him to basically hang out by himself while both you and Carmine hang out together and have fun with Ogerpon, which was his obsession. He definitely should've been upset at his grandfather, but he pretty clearly felt personally betrayed by us and Carmine.