Spinel Sun functions as one of Eriol Hiiragizawa's magical guardians, designed as a counterpart to Cerberus. His typical borrowed form appears as a small black cat featuring butterfly-like turquoise wings and eyes. His true form manifests as a larger black panther with prominent butterfly wings, a turquoise gem on its forehead, and silver butterfly-themed armor adorning its chest. Personality-wise, Spinel exhibits a calm, refined, and quiet demeanor, favoring reading and peaceful environments—a direct contrast to Cerberus's energetic nature. He dislikes loud noises and possesses a deliberate vulnerability to sugary foods; consuming sweets triggers an intoxicated state marked by frenzied gluttony, emotional outbursts, and uncontrolled energy beam attacks. His abilities include winged flight, supersonic speed, projection of crimson energy bursts, and control over solar energy and light, drawing strength from the sun.

In the theatrical short "Leave it to Kero! Theatrical Version," Spinel shares takoyaki with Cerberus. After cautiously confirming the savory snack lacks sugar, he eats. When only one piece remains, a dispute escalates, accidentally launching it out the window. Spinel and Cerberus engage in a lengthy, chaotic chase across town to retrieve it. During the pursuit, Spinel lands on a cake, triggering his sugar-induced frenzy and causing further mayhem until the takoyaki is lost. Exhausted, they return home. Cerberus, moved by Spinel's sleepy admission of liking both sweets and takoyaki, leaves him the last piece brought by Sakura's father.

Beyond this short, Spinel appears throughout the series. He assists Eriol in observing Sakura during the Clow Card arc and participates in the final battle against Cerberus, acknowledging the power of their opponents' bond with Sakura. In the Clear Card arc, following Cerberus and Yue's weakening from the loss of the Sakura Cards, Spinel and Ruby Moon relocate to Tomoeda. They stay near the guardians to channel Eriol's magic, supporting them during Eriol's recovery. Spinel resides at Sakura's house with Cerberus, collaborating to investigate the Clear Cards and encountering Momo, the guardian of a mysterious new book. His relationship with Cerberus evolves from initial rivals to respected allies and companions, demonstrated through shared meals, correspondence, and collaborative efforts in later storylines.