Mamoru Iwashima is a classmate of Kiyo Takamine at Mochinoki Second Middle School. He is described as short in stature with a somewhat overweight build, and his hair is spiky, resembling a burr. Kiyomaro himself has referred to Iwashima as an interesting boy. Despite his somewhat comic appearance, Iwashima is a genuine member of the school's Mysterious Mystery Research Club, indicating a natural curiosity about unusual phenomena.

Initially a background character within the series, Iwashima gradually assumes a more prominent role as the story progresses. He is one of Kiyo's high school friends and becomes increasingly involved in the events surrounding the mamodo battle, though he does not possess any magical abilities or serve as a book owner himself. His motivations are rooted in his friendship with Kiyo and his growing awareness of the extraordinary circumstances occurring around him.

Iwashima's relationships are primarily centered on Kiyo and the other classmates. He provides a sense of normalcy and comic relief, often reacting to the strange situations that arise with bewilderment or humor. Over the course of the series, he develops from a peripheral observer into a more engaged participant, showing loyalty and support for Kiyo during key moments. His notable trait is his inquisitive nature, which aligns with his membership in the mystery club, though he possesses no special combat abilities. His development highlights his increasing willingness to step outside his ordinary school life to help his friends.