@jak on the contrary, he's really too nice I think. He keeps on warning Shuusei and he doesn't do anything behind his back. Reon's feelings are genuine and he's not dissing Shuusei in front of her. He tries to win her over on his own merits.
And Reon is right: people's feelings change. If Shuusei doesn't make any effort, she might drift away from him.