Mimirin Midorihara is a white anthropomorphic rabbit featuring black ear tips, a pink nose, a visible buck tooth, and a forehead tuft of hair. Her appearance evolves across series installments. Initially, she wears a reddish-pink flower-shaped hair clip, a pastel yellow sweater, a red overall dress, white socks, and light red shoes. Later, her outfit changes to a pink flower clip with a pink jacket, pastel yellow shirt, pastel blue skirt, and pale pink socks. In recent iterations, she primarily wears a pink ribbon next to her left ear—a gift from her childhood friend Ramurin before Ramurin's departure from Challenge Island—alongside a grey polka-dotted dress, a pink cardigan, blue striped socks, and magenta shoes with white soles. Seasonal variations include summer attire like a pink sleeveless dress with a lighter ribbon and grey shorts.

Her family runs the Flower Shop Midorihara on Challenge Island. As an only child, her status was emphasized after Ramurin's departure left her the sole main character without siblings. Mimirin exhibits a gentle, naive, and somewhat spoiled personality. She is lively yet clumsy, occasionally selfish, and prone to crying when others disagree with her. Despite this, she demonstrates bravery in forming new friendships and commitment to overcoming challenges. She enjoys racing, where she is the fastest among her peers, and has interests in dolls, flowers, and singing. Her career aspirations include nursing and floristry.

Her preferences and fears are distinct: her favorite food is strawberry shortcake, contrasting with her dislike of carrots—a trait highlighted as unusual for a rabbit but rationalized by the potential harm of carrots in large quantities. She fears snakes, caterpillars, and other insects. Relationships are central; she is a close friend of Shimajiro Shimano, whom she met when he was shy, and shares bonds with Torippi Sorano, Nyakkii Momoyama, and kindergarten peers like Marurin Sasaki and Sakurako Koinuma. Her friendship with Ramurin Makiba is particularly significant due to Ramurin's move to France resulting in the keepsake ribbon.

Additional details include her birthday on October 10, blood type B, and a tendency in the original Japanese version to refer to herself in the third person using the verbal tic "-nya" at sentence endings. Her surname, Midorihara, means "green field," while "Mimirin" derives from "mimi" (ears) and "rin" (dignified), referencing her rabbit ears.