Princesa Neptina originated in the Japanese anime series initially developed as "Dolphin Prince" before evolving into "Marine Boy." She debuted as Neptuna in the 1965 black-and-white pilot "Dolphin Prince," serving as the protagonist's companion. Following the series' rebranding into color animation as "Undersea Boy Marine" (commonly called "Marine Boy"), her name standardized to Neptina.

Physically, Neptina appears as a young bare-chested mermaid whose long, flowing hair strategically preserves modesty by covering her upper body. Her lower body features a yellowish-green dolphin-like tail, establishing her as an unscaled merfolk variant. She consistently wears a distinctive magical pearl necklace. This pearl generates protective force fields against dangerous sea creatures, projects crystal ball-like visions of future or distant events, and creates defensive barriers during confrontations. Neptina frequently employs these abilities to aid Marine Boy and the Ocean Patrol.

Neptina exhibits an innate connection to marine life, intuitively understanding and communicating with sea creatures. She clearly comprehends Marine Boy's white dolphin companion Splasher, while her insights into other aquatic animals often prove crucial during Ocean Patrol operations. Throughout narratives, she repeatedly disobeys Marine Boy's directives to avoid conflict zones, instead intervening in perilous situations to rescue him or team members. This pattern establishes her independent courage and pivotal role in mission success.

Her series role centers on supporting the Ocean Patrol's objectives as Marine Boy's primary ally. She contributes unique abilities to underwater endeavors, consistently demonstrating loyalty, resourcefulness, and a willingness to challenge protective mandates when necessary.