Ceres is one of three legendary Mashin, or Rune Gods, ancient guardians of Cephiro. These sentient, god-like beings manifest as colossal mechanized constructs piloted by chosen Magic Knights, though they are living entities. Ceres embodies water and bonds deeply with its pilot, Umi Ryuuzaki.

Initially dormant, Ceres appears as a massive blue dragon with a corpulent, winged, four-legged form inspired by Western mythology, residing in a pocket dimension. Umi journeys to the Sea Temple to awaken it, facing a trial where Ceres tests her resolve to protect friends against threats like Ascot's beasts. Upon acceptance, Ceres transforms into its primary battle form: a humanoid mecha resembling a robotic knight. Umi pilots from within, movements synchronized through a psychic link enabling telepathy but transferring damage sustained by Ceres to Umi as physical strain. Damage to Umi's sword or her unconsciousness causes Ceres to vanish back to its dimension.

In combat, Ceres wields water-based magic, amplifying Umi's abilities. Its signature techniques include destructive light beams from its fists and high-speed offensive tactics reflecting Umi's assertive style. Created by Cephiro's deity, Mokona, Ceres is a primordial Mashin, distinct from artificially created ones like Zagato's or Emeraude's which lack independent consciousness. Alongside the fire-aligned Rayearth (piloted by Hikaru) and wind-aligned Windam (piloted by Fuu), Ceres can merge to form the exponentially more powerful Combined Rayearth (Gattai Rayearth). This unified entity combines their attributes—Rayearth's shield, Windam's wings, Ceres' tail—deployed against existential threats like the corrupted Pillar Emeraude or the anime-exclusive antagonist Debonair's Regalia.

The OVA adaptation differs: Ceres' dormant form becomes an Eastern-style serpentine dragon with small legs and no wings, summoned when Umi accepts its pact directly during an attack on Tokyo. This iteration is sometimes named "Lexus." Ceres also appears in the CLAMP multiverse crossover *Tsubasa: Reservoir Chronicle* as a Kudan—a protective spirit resembling a blue dragon assigned to Kurogane.

Naming conventions vary across translations: "Ceres," "Selece," "Seles," "Celes," and "Seresu" stem from differing interpretations of the katakana. Merchandise and production notes suggest "Ceres" is intended, possibly inspired by the Toyota Corolla Ceres or Kia Ceres vehicles.

After resolving Cephiro's conflicts, Ceres departs with Mokona and the other Mashin to traverse other dimensions.