Mayuko Satō is a supporting character in the anime Just Because!. She is one of Mio Natsume's best friends at Kashiogawa High School. Mayuko is a third-year student, like the main group, and is seen attending the same prep school as Mio while preparing for entrance exams. She has long blonde or brown hair and yellow or brown eyes, depending on the source, and is described as a coquettish and fashionable teenage girl.

Her personality is defined by deep loyalty and protectiveness toward Mio. Mayuko is extremely attached to her friend and becomes hostile toward Hazuki Morikawa because she fears that Hazuki may "steal" Haruto Sōma away from Mio, whom Mayuko believes has feelings for Haruto. This fierce protectiveness drives her to be openly antagonistic toward Hazuki, though it stems from a desire to safeguard Mio's happiness. Mayuko also exhibits a go-getter attitude, taking active steps to influence the romantic dynamics around her and to defend her friend's interests.

Within the story, Mayuko serves primarily as a satellite character who closely shadows Mio and speaks out on her behalf. Her presence adds tension to the love triangle involving Mio, Haruto, and Hazuki, as Mayuko constantly tries to steer events in Mio's favor. She has little direct interaction with the male leads and her development is minimal, remaining largely static as a loyal friend who acts as a barrier between Mio and the potential threat of Hazuki. Mayuko does not display any special abilities beyond her strong interpersonal loyalty and her willingness to assert herself in social situations to protect her close friend.