Responses
Not 100% sure, but probably because he's surprised with Chiharu kissing his forehead. Affection is a foreign concept to him, at least in terms of receiving affection. Chiharu hums questioningly at him the panel before he sits up but he doesn't have the words to say what's on his mind outright, so he just says 'No' as if to say 'It's nothing.'
Why does Rin Sama serpent water god say "no" after smexy times in chpt 9?