Piro is a stray cat which they take in despite Nayuki's allergies, Makoto becomes very attached to the cat and you often see Piro on her head. Piro's name derives from 'Piroshki' (Пирожки [rus]), a Russian word for meat buns - that name was suggested by Yuuichi as a joke but Makoto shortened it to Piro although she had no idea what it meant.