Prince Justin rules the coastal kingdom of Gandor. His first encounter with Marina unfolds during a violent storm and shipwreck caused by a three-headed sea serpent, where he nearly drowns before her rescue. This incident forges a connection between them, though Justin initially misattributes the rescue to his arranged fiancée, Princess Cecily. The misunderstanding leads to an engagement founded on gratitude, not affection.

Upon discovering Marina’s true role in saving him, Justin ends the engagement to Cecily. He develops a deep romantic bond with Marina, navigating the challenges of her mermaid nature and his human royal duties. Their relationship thrives through magical interventions, notably a potion from Anselm—Justin’s tutor and a secret wizard—that temporarily grants Justin underwater breathing, enabling clandestine meetings beneath the ocean. In extended narratives, Justin actively collaborates with Marina’s underwater allies, including Winnie the seahorse, Bobo the tropical fish, and Ridley the sea otter, to confront threats from antagonists like the sea witch Hedwig and Prince Lothar of Brakston, Gandor’s rival kingdom.

Justin’s background features his parents, King Charles and Queen Catherine, with the latter particularly invested in his romantic prospects. His closest human confidant, his page Chauncey, aids in sustaining his relationship with Marina. Overcoming obstacles requires cooperation between sea and land dwellers, fostering broader acceptance of merfolk-human relations. Justin’s journey culminates in gaining approval from both kingdoms to marry Marina, facilitated by her magical transformation into a human. Their union symbolizes reconciliation between their worlds.

Later storylines introduce their daughter Manila, extending the narrative of their family and the enduring connection between oceanic and terrestrial realms.