A boy who becomes interested in and joins Ishida's group of friends upon learning they are making a movie. He is eager to be the leading actor and is considered handsome and talented enough to do so. Mashiba was lonely as a child and the other children took him for granted and bullied him, now he is completely intolerant of bullies in any form, and while generally calm and level-headed, he reacts violently to any random act of bullying he sees.
He is oblivious to Kawai's crush on him until Ishida points it out. It is later revealed that Mashiba befriends Ishida because of his social awkwardness and never being able to fit in at school despite trying to; by hanging out with Ishida—believing him to be even stranger than he is—he would appear normal. However, this only makes him feel that he should be the least important amongst the group. Thus, when Ishida asks Mashiba to hit him after the former reveals his bullying records, he calmly does so. Mashiba is the first to reconcile with Ishida upon his release from hospital and later studies to become a teacher after graduation.