Mizuki Hayama is a middle school student at a school affiliated with Otobane Academy in Otowa, Japan. She appears in ef: a tale of memories as a supporting character and becomes one of the two main heroines in ef: a tale of melodies. During the winter break depicted in melodies, she travels to Otowa, Australia.

Mizuki is cheerful, outgoing, and notably blunt. She speaks frankly about personal matters without hesitation, particularly with her admired senior, Kei. She is a member of her school's girls' basketball team and greatly enjoys reading shōjo manga, including works by an author named Hiro Hirono.

Her primary motivation in the story is her deep affection for Shuuichi Kuze, a man she first met as a child. After hearing him play the violin, she becomes determined to draw closer to him despite his persistent attempts to keep her at a distance. Her attraction to Kuze and her refusal to be discouraged form the central emotional thread of her role in the narrative.

Mizuki's key relationships are with Kei, whom she looks up to and treats with direct honesty, and with Kuze, whose reserved and tragic past she seeks to understand. Over the course of melodies, she develops from a straightforward, spirited girl into someone willing to confront emotional vulnerability and the weight of another person's hidden pain. Her notable ability lies not in any supernatural talent but in her emotional openness, persistence, and the way her presence challenges Kuze's isolation. Her appreciation for music, especially the sound of his violin, deepens her connection to him.