Marin is a mermaid distinguished by pink hair, dark red eyes, a light blue hair accessory, and a red tube top. Her red tail fades to light pink at the fin. Exposure to air transforms her tail into legs. Her personality is marked by selfishness in interactions. She harbors a strong romantic interest in Riiya, unaware of his werewolf nature despite their frequent encounters. She maintains a persistent rivalry with protagonist Chacha, primarily over Riiya's attention.

She attends Urara School in the Apple class taught by Teacher Barabaraman but frequently interacts with students from the Banana class, including Riiya. Her social connections include occasional, situational alliances with Yakko and Orin. Marin debuted during a beach episode where she first met Riiya, sparking her ongoing infatuation. This dynamic cements her role as Chacha's romantic rival, embodying the "Veronica" archetype opposite Chacha's "Betty."

Marin participates in major story arcs, notably joining the quest to defeat Daimaou. She unites with Chacha, Riiya, Shiine, Yakko, Orin, Seravy, and Dorothy as one of the eight Holy Warriors of Love, Courage, and Hope. This alliance storms Daimaou's castle to restore the kingdom, requiring her cooperation with the group despite her earlier antagonism, though her core traits persist.

Marin occasionally breaks the fourth wall, switching to a performative "nice girl" persona when observed, acknowledging the shojo anime context. The sequel manga *Akazukin Chacha N*, set in modern Tokyo, does not incorporate her character. Her established role across the anime, OVAs, and spin-offs lacks explicit development, additional backstory, or expanded family connections.