Umi Ryuuzaki, also known as Marine in some localizations, is one of the three main protagonists of Magic Knight Rayearth, representing the water element as a Magic Knight. She is a fourteen-year-old eighth-grade student from Tokyo, the only child of wealthy, doting parents, which gave her a privileged upbringing. She attends an exclusive private school and serves as captain of her fencing team. Physically, she is tall with long, flowing blue hair and blue eyes.

Her personality is layered. Outwardly, she appears elegant and composed, but she has a short temper, is quick to speak her mind, and can be blunt and sarcastic. She is initially the most reluctant of the three Magic Knights, skeptical of the strange world of Cephiro and anxious about missing a fencing competition. She often panics and complains, and she has a running feud with the creature Mokona, whose behavior frustrates her. However, beneath this prickly exterior, she is deeply loyal, protective, and compassionate. She gradually adopts the role of the group’s big sister, especially toward Hikaru and Fuu, and she is willing to risk her life for them. She tries to hide her tender side and may deny her good deeds, a trait that reflects a tsundere-like personality.

Umi’s primary motivation shifts over the course of the story. At first, she wants only to return home to her comfortable life and her fencing career. As she bonds with Hikaru and Fuu, her motivation becomes the protection of her friends and the fulfillment of their quest to save Cephiro, even when that quest forces them to face a heartbreaking truth about the Pillar, Princess Emeraude. In the second arc, she struggles with guilt over their actions, but her sense of duty and loyalty drives her to help defend Cephiro and support Hikaru’s ultimate decision to end the Pillar system.

Her role in the story is that of the water-wielding Magic Knight, providing both combat strength and emotional grounding. She often serves as the voice of reason, calling out absurdities and keeping the group focused. She also supplies comic relief through her reactions to Mokona and her outbursts.

Key relationships define her growth. She forms a close sisterly bond with Hikaru and Fuu; she is protective of Hikaru and looks up to Fuu’s calmness. She constantly bickers with Mokona but cares for it. In the anime, she harbors a crush on the sorcerer Clef, who regards her only as a friend, while the young mage Ascot secretly has feelings for her, though she remains oblivious. In the manga, her connection with Ascot is more developed, hinting at a budding romance. She also befriends the Chizetan princesses Tatra and Tarta after dueling them.

Umi undergoes clear character development. She begins as a spoiled, impatient, and self-centered girl, but her experiences in Cephiro force her to mature. She becomes more selfless, controlled in battle, and emotionally open. By the second series, she is more composed and experienced, though she still retains her sharp tongue. Her greatest inner change is learning to rely on others and to overcome her fear of being alone.

Her notable abilities stem from her expertise in fencing and her water-based magic. She wields a rapier, initially a thin sword and later the mystical Rapier of Escudo, which turns to water if anyone else touches it. Her magical spells include Water Dragon, which summons a large dragon-shaped torrent of water; Sapphire Whirlwind, a defensive and offensive water tornado; and Ice Blade, a rain of icicle projectiles. She pilots the Mashin (Rune God) named Selece, a blue, bat-winged sea dragon, which embodies her will and spiritual growth. Her armor evolves over time, culminating in a black leotard with blue armor and a cape.