Masato Iwai is a supporting character in the series Super GALS! and its source material Gals!. He is the longtime boyfriend of Sayo Kotobuki, having been in a steady relationship with her since the beginning of junior high school. Their romance began when he noticed her as the cute girl who always wore a hat to school, a habit she adopted because she had cut her bangs too short and was self-conscious about it. From that point onward, the two became an inseparable couple.

As a person, Masato is described as fun-loving and somewhat quirky, traits that align closely with Sayo's own personality. He shares her enthusiasm for detective work and frequently joins her in playing the roles of junior detectives, a pastime that often draws attention from those around them. Despite this playful and eccentric side, Masato possesses a strong sense of justice and gallantry, particularly where Sayo is concerned, and he is consistently portrayed as brave and daring. He has a clear determination to join the police force when he grows up, indicating a serious and goal-oriented side to his character that balances his more lighthearted tendencies. In his interactions with the main cast, Masato tends to be helpful and supportive, fitting the role of a reliable friend and partner. He does not undergo major dramatic transformations over the course of the narrative, but his steady presence and unwavering loyalty to Sayo form a consistent and dependable part of the story's ensemble dynamic.