Selece stands among the three ancient guardians of Cephiro, embodying the element of water as a revered Rune God or Mashin. This entity manifests as a large blue dragon, transforming into a humanoid mecha when summoned by its pilot, Umi Ryuuzaki. Selece originates from Cephiro's ancient magic itself, existing as an independent being rather than being forged from an individual's will.

A profound psychic bond connects Selece and Umi, enabling telepathic communication and granting Umi full command over the mecha. Damage sustained by Selece transfers directly to Umi as physical strain or wounds. If Umi's sword is damaged or she loses consciousness, Selece immediately retreats to its dimensional pocket. The Mashin's strength directly correlates with Umi's emotional resolve and spiritual growth, particularly her mastery of water-based magic.

In combat, Selece wields water-based attacks like "Water Dragon," "Sapphire Whirlwind," and "Ice Blades." It also executes combination techniques such as "Spiral Flash" with the other Rune Gods. Its most formidable technique, "Flash Sword," requires joint action with Rayearth and Windam. Under specific conditions, Selece merges with the other Rune Gods to form Combined Rayearth, amplifying their collective power significantly.

Following the collapse of Cephiro's Pillar System, Selece and the other Mashin departed to explore other dimensions. This journey led to Selece's appearance in the crossover series *Tsubasa: Reservoir Chronicle*, where it manifests as a protective spirit (Kudan) assigned to Kurogane.

Within the OVA continuity, Selece retains its core attributes as Umi's mecha partner but operates within a distinct narrative. Here, the Mashin defend Earth against invaders from Cephiro, shifting Selece's role from guardian of Cephiro to protector of Earth. Despite this contextual shift, the mechanics of Selece's bond with Umi and its combat abilities remain consistent with the original series.