Midori Kasugano is the main female protagonist of the anime and manga series Midori Days. She is a sixteen-year-old high school student who comes from a wealthy and prestigious family, living in a large mansion with many servants. Despite her privileged background, she is an extremely shy and emotionally fragile girl who struggles to talk to others, particularly around the object of her affection. For three years, she has harbored a secret and intense crush on Seiji Sawamura, a notorious delinquent from a different school who is feared by many but known to protect the weak. Because of her crippling shyness, she has never been able to confess her feelings to him.

Midori's quiet life is turned upside down when a supernatural event causes her to shrink to a miniature size and become attached to Seiji's right hand in place of his normal hand. Her real body falls into a coma, and her mother desperately tries to find a way to wake her. As a tiny figure living on Seiji's hand, Midori is forced into constant, intimate proximity with him. This strange situation acts as a catalyst for her character growth. Initially, she is timid and prone to tears, often feeling like a coward. However, the forced closeness gradually allows her to open up and become more vocal and assertive. She repeatedly gathers the courage to confess her love to Seiji, even when her feelings are not immediately returned. She also displays a jealous and competitive side when other girls show interest in him, but over time she learns to appreciate the good qualities of even her rivals.

Midori's primary motivation throughout the story is her deep and unwavering love for Seiji. Her role in the narrative is to be both his constant companion and his emotional anchor. While she cannot physically fight or perform combat, her small size allows her to be with him at all times, offering support, advice, and a stabilizing influence on his otherwise rough life. She participates in his daily activities by riding on his hand, and she occasionally demonstrates creative insight that helps him in difficult situations. Although her powers are not combat-oriented, she sometimes wakes up in her full-sized body for brief periods, only to return to her comatose state when feelings of loneliness overwhelm her, underscoring the connection between her emotional state and her physical condition.

Her key relationships include her deep bond with Seiji, which evolves from a one-sided crush into a mutual, albeit complicated, connection. She also has a family tie to Haruka Kasugano, her mother, who is deeply worried about her coma. Among her rivals is Shiori Tsukishima, who also competes for Seiji's attention, and a childhood friend named Kouta Shingyouji, who has his own feelings for Midori. Throughout the series, Midori's development is marked by her growing confidence and her gradual understanding that the world continues to exist beyond her feelings for Seiji. She learns to confront her fears and assert herself, evolving from a shy admirer into a more determined and resilient individual who plays a central role in Seiji's own growth toward maturity and responsibility. Her notable characteristic is her transformation into a miniature form, which, while a limitation, becomes the unique medium through which her relationship with Seiji unfolds.