Hippo is a central supporting character who serves as the guardian and guide for the mermaid princess Lucia. Initially appearing in the human world as a small, blue penguin wearing a white sailor uniform and a matching hat with a red ribbon, this is not his original form. His true identity is that of a Hippocampus, a legendary creature resembling a unicorn or a seahorse from Greek mythology. He was originally a hippocampus before the key that allowed him to transform was stolen, which forced him to remain primarily in his penguin form for much of the story. Later in the series, he also gains the ability to transform into a handsome human boy with blonde hair and light green eyes, wearing an outfit similar to his penguin clothes.

In terms of personality, Hippo is portrayed as highly polite and formal, consistently using respectful honorifics when addressing others, including the mermaid princesses, even when he is angry or frustrated. He is deeply caring and protective of Lucia, as well as the other mermaid princesses Hanon and Rina, though he often finds that they do not listen to his advice. He can be strict, particularly when it comes to rules or situations that might cause harm, and he initially voices strong disapproval of Lucia's forbidden relationship with a human boy named Kaito. Despite his self-proclaimed humility as a servant, he possesses a somewhat vain side, considering his penguin form to be quite handsome. He also has a voracious appetite and is frequently the victim of slapstick misfortune, often being hit or thrown across a room by the other characters.

Hippo’s primary role in the story is to accompany Lucia to the human world to aid in her search for her missing pink pearl, which she lost. He possesses a Pearl Radar, a device that can track the location of the mermaid princesses' pearls. Beyond this, he acts as a source of knowledge about the mermaid world and serves as a protector. When the princesses face danger from enemies, Hippo can transform into his true hippocampus form or a guardian form using the Key of Sealing. In these forms, he wields significant power, using the key as a sword or a staff to perform magical attacks and destroy the enemies' singing stages to defend the princesses.

His most significant personal relationship develops when, in his human form, he falls in love with Yuri, a villainous Dark Lover working for the series' antagonist. Despite their opposing sides, he gives her a pair of sandals after accidentally injuring her bare foot, and their feelings for each other grow. However, their relationship ends tragically. Hippo makes the difficult decision to part ways with Yuri, believing that his duty to protect the mermaid princesses must come first and not wanting to put them in danger. His feelings for her do not fade, and he continues to love her after their separation. He later gets a chance to see her again briefly, providing some closure to his romantic arc.

Throughout the series, Hippo undergoes notable development. He begins largely as a rule-enforcing mascot character but gradually becomes more compromising as he personally experiences the power of love that he once criticized Lucia for chasing. He learns to accept his human form, which he initially disliked, and embraces his role as a powerful warrior, fully transforming into his true hippocampus form when he finally retrieves the stolen sealing key. His journey involves balancing his duty as a guardian with his own personal desires and heartbreak. Beyond his transformations and combat abilities, his other notable skills include using his pearl radar and wielding the Key Rod Release magical attack. He continues his role as a caretaker and protector into the sequel series, where he is also seen looking after Lucia's daughter.