Marina Ryuzaki, also known as Umi Ryuzaki, is the Magic Knight of Water in the second arc of Magic Knight Rayearth. She is a fourteen-year-old student from a wealthy family of doting parents and the only child. At the beginning of her journey, she presents as elegant and beautiful with long blue hair, but she has a short temper and a somewhat spoiled, uncaring attitude. She is the most reluctant of the three Magic Knights, initially preoccupied with an upcoming fencing tournament in her world and skeptical of the strange land of Cephiro. However, beneath this prickly exterior lies a deeply loyal and protective nature, especially toward her fellow knights Hikaru and Fuu. Over time, she matures notably, shedding much of her initial selfishness and becoming a reliable "big sister" figure who looks out for her friends. In the second story, returning to Cephiro a year later, she carries lingering guilt from the events of the first journey but shows greater control, experience, and sweetness.

Her motivations shift from a desire to return home quickly to a genuine commitment to saving Cephiro and supporting her companions. She plays a key role in defending the remaining land from invaders, particularly the forces of Chizeta. She develops a rivalry with the twin princesses Tatra and Tarta, dueling them in combat and eventually forging mutual respect and friendship. In the anime, she holds a romantic interest in the mage Clef, though it is unreciprocated, while the young summoner Ascot harbors unspoken feelings for her, which she often remains unaware of. Her relationship with the creature Mokona is comically antagonistic; she is frequently annoyed by its behavior, yet they share a bond of companionship.

Marina wields water-based magic, with spells such as Water Dragon, Ice Blade, and Blue Tornado. Her weapon is a thin rapier forged from the mineral Escudo in Cephiro, and her skill as a fencer from the real world translates directly into her combat style. She also pilots the Rune-God Selece, also called Ceres, a blue dragon-like Mashin that magnifies her power. Her growth throughout the adventure is among the most pronounced, transforming her from a reluctant, hot-tempered girl into a mature, compassionate, and formidable warrior who places the safety of her friends above all else.