Aqua Regina is referred to by Nikora (and later, Fuku-chan) as "the Goddess of the Ocean World." Aqua Regina was once a powerful being, but by the time that the manga series begins, she is nothing more than a ghostly specter haunting the ocean. Why? The flickering, ghostly visage of Aqua Regina appears to Lucia in chapter 06, and explains that in the past, when the Panthalassa clan threatened the Earth, she used all of her power in order to seal them within the deep ocean. (In chapter 17, Gakuto himself confirms that this happened tens of thousands of years ago.) Aqua Regina depleted her power in doing so, however, which is why she now exists only as a ghostly spirit.
In order to fully revive Aqua Regina, the seven mermaid princesses must combine together the power of their seven pearls. And reviving Aqua Regina is the only way to stop Gakuto this time around. Gakuto further explains that once the seal upon the Panthalassa is broken once, it can never be repaired again without Aqua Regina's power. And the seal is broken, thanks to Sara.
Until the seven princesses could gather together again, the ghost of Aqua Regina remained protecting the kingdom of the North Pacific Ocean, the last remaining mermaid kingdom, from Gakuto's attacks. In chapter 20, when Aqua Regina is finally called forth by the mermaid princesses again, she repairs the seal on the Panthalassa clan, and sends Gakuto (and Sara, by her own choice) down to the bottom of the ocean. There, she says, "Gakuto's ocean will embrace them both in a peaceful sleep." After that, Aqua Regina vanishes.
In the second story arc of the manga, after Seira is born and the seven mermaid princesses can combine their song once again, Aqua Regina is, yet again, summoned. This time around, however, Aqua Regina does not play much of an active role in the final battle, save to give words of encouragement to the mermaid princesses. She may or may not be responsible for summoning the "ancient ones" that appear to save Michel - the manga leaves that bit unclear. However, it is Aqua Regina who performs the final miracle that returns baby Michal to Rihito.
Hippo is simply known as Hippo, and that's that. Personally, I always thought that it was a rather unfortunate name, or perhaps even an allusion to Hippo's, er, pudginess. But the name makes sense once you read chapter 17...
Image: 'Hippocampus' written in Japanese characters.
In chapter 17, Hippo's true name, Hippocampus, is revealed. A hippocampus is a creature from 
Greek mythology with the head, torso, and front legs of a horse, but with a fish tail for the lower part of its body. Poseidon's chariot is said to have been drawn by a hippocampus.
Hippo does not have an official profile.
Hippo
Hippo is Lucia's guardian penguin. From the beginning of the manga, he doesn't seem like much of a useful guardian. Hippo is basically used as slave labor around Pearl Waters - he cooks, cleans, does the laundry, and constantly nags at Lucia. He serves only two useful functions. One is that he has the Pearl Radar, which can detect the presence of the pearls of the other mermaid princesses. The other is that he is an important source of knowledge about the mermaid world, and he and Nikora often find themselves lecturing Lucia about things that she should already know.
Hippo is your typical, old-fashioned Loyal Retainer in every sense of the word. He speaks very politely, even when he's angry at Lucia, he is always careful to observe the social distance between himself, a mere servant, and Lucia, a royal princess. Hippo was most likely raised around royalty, and was probably trained to be a servant from the moment of his birth.
Despite all of his self-professed humility, however, Hippo is actually quite full of himself. He considers his penguin body to be quite handsome, and the extreme lack of attractive female penguins in and around Japan is a constant source of misery for him.
Hippo seems to have born beneath an unlucky star, which would explain why in almost every chapter he ends up being punched by Lucia, smacked in the head with a towel, assaulted by other penguins, or thrown across a room by Kaito. Hippo is quite possibly the most unfortunate penguin ever to grace the surface of the Earth (or the floor beneath the ocean, for that matter).
It is only much later in the series that the secrets of Hippo are revealed. First he can assume a human form, and second, he can assume the shape of a mythical Hippocampus. The Hippocampus is Hippo's "true" form, although at the same time, the Hippocampus seems to have a different mind, and a different personality, than the Hippo that we normally know and love. In his simple human form, however, Hippo is still very much the same person that he always is, and he also has the command of substantial magical powers.
Hippo's Forms
penguin Hippo
This is Hippo's most basic shape, and also his most powerless form (first appearance: chapter 01).
human Hippo (1)
Hippo transforms into a human because he's "been living on dry land for too long." Because Hippo initially bursts out of his penguin clothes and is left naked, Rina and Hanon decide to dress him in an adorable sailor suit. Afterwards, every time that Hippo assumes his human form, he is automatically dressed in the same sailor suit.
human Hippo (2)
Hippo appears in this form during the final battle within Gakuto's castle
Hippocampus
Hippo's true form, and "the sacred guardian beast of the gate of the seal"
Hippo's Powers
transformation
Hippo can transform himself from a penguin into several different shapes.
wings
In his human form, Hippo has a pair of tiny wings on his back, normally kept hidden beneath his shirt. However, when Hippo is threatened or in danger, his wings burst out of his back and unfurl to a much larger size.
"Key Rod Release!"
Hippo's magical attack.
Magical Items
Pearl Radar
I don't know if this counts as "magical" or not, but whatever. Hippo uses the Pearl Radar to detect the presence of the pearls of the mermaid princesses. However, he drops the Pearl Radar in chapter 11, and Caren picks it up. Caren loses the Pearl Radar to the Black Beauty Sisters in chapter 13. Finally, in chapter 15, Sara crushes the Pearl Radar with her bare hands.
key to the seal
This was the seahorse-shaped key that Sara used to unlock the seal placed on Gakuto's clan. When the key is "awakened" in chapter 17, it plays a sad, somber melody, and sprouts wings. The awakening of the key allows Hippo to transform into his final form, Hippocampus.
key rod
When Hippo appears inside Gakuto's castle in chapter 19, his key has been transformed into a powerful rod. Hippo uses this rod to perform his magical attack, Key Rod Release. Later, Hippo hands the rod over to Hanon. She, of course, uses it like a baseball bat to break down a door.
Kaito, the son of a pair of musicians, is an avid surfer, as well as Lucia's love interest. Though he is popular with girls, he is secretly in love with a beautiful mermaid who saved him from a shipwreck seven years ago, which killed his parents, and has yet to realize that this mermaid is Lucia. Later in the story, it is discovered that beneath Kaito's human exterior lie some distinctively not human qualities.
Doumoto, Kaito
Kaito is a junior high school student, in the same class as Lucia. He is strong, handsome, a phenomenally talented surfer, and widely workshipped by girls of all ages - and he knows it, too. Kaito's ego, however, may be largely a front that hides his feelings of fear and insecurity. In fact, in chapter 17, Kaito even says that surfing is the only thing that lets him truly feel like himself. His parents, both famous musicians, died when he was young, and he has been living on his own since then. Since Lucia appeared in his life, Kaito has been having frightening dreams, a glowing light sometimes appears on his forehead, and he occasionally exhibits strange powers. Kaito's greatest fear is that he might not actually be human.
Kaito was saved from drowning by a mermaid seven years ago, and he has spent his life searching for that mermaid. Now that he's seen her again, he has become obsessed with his mysterious mermaid. At the same time, however, Kaito has grown increasingly fond of this cute, sweet girl in his class, named Lucia. Now Kaito also fears that his heart will eventually have to choose between Lucia and his beloved mermaid.
As if Kaito didn't have enough problems, between his love life, his mysterious splitting headaches, the bad dreams, and the monsters attacking him, he receives another bad shock in chapter 16. Kaito finds a letter left behind by his parents, written shortly before they both died. The letter reveals that Kaito was never their real son - instead, they found him as a baby, washed up one morning on the beach, wrapped within a mysterious light. Kaito's parents knew from the beginning that he was not a normal child, but they never had the heart to tell him face-to-face.
Kaito is actually one of the two twin princes of the Panthalassa clan. His slightly elder twin brother is none other than Gakuto himself. The seal that Aqua Regina placed on the Panthalassa clan would not even allow the birth of twins, for fear that if the two of them combined their powers, the Panthalassa would once again menace the ocean world. Thus, shortly after twin princes were born, a mysterious force flushed baby Kaito out of the castle, up through the ocean, and onto a beach in Japan.
After Kaito comes to terms with his true identity, and with Lucia's little secret, he is finally able to live happily and confidently, the way that he always wanted to. However, Kaito is still plagued by self-doubt; one of the reasons that he continues surfing, as he confesses to Lucia, is that only when he is riding the waves can he really find himself.
Kaito is always afraid that because of his lineage, he will somehow end up hurting the people that he cares about; and indeed, his worst nightmares come true during the second story arc of the manga. A sweet, shy, lonely girl named Michal falls in love with Kaito; Kaito considers her a true friend, but ultimately has to break her heart and tell her that he loves Lucia instead. When Michal is subsequently consumed (literally) by grief, Kaito is almost overwhelmed by pain and guilt. "The sadness of not understand who - what - you really are... The pain... Realizing that you're an inhuman monster that may never be loved by anyone... Michal suffered the same way that I did back then... I'm the one who should have understood her the most, and I'm the one who caused her to suffer the most!" he cries out, in a burst of self-loathing. Ultimately, however, a sound scolding (and a punch in the face) from Rihito gives Kaito the courage to get back on his feet.
No matter what happens, Kaito will always be there to protect Lucia. Kaito has many personal and emotional burdens to shoulder, but he knows that he can rely on Lucia for support - and he knows that he has to keep smiling for her sake, too. In the end, nothing is capable of keeping Kaito down. His cocky grin, and the mischevious twinkle in his eyes, still manage to shine through, no matter what sort of ordeals he has to endure.
Kaito's Forms
Human Form
Prince Form
Kaito appears this way, dressed as a prince, when he is pulled into the worlds beneath the ocean surface. Granted, in the image linked above, "prince" Kaito happens to be in mortal peril. But still, it was the best image of his princely attire that I could find.
Kaito's Powers
light
When Kaito is threatened or in danger, a glowing sigil appears on his forehead. This light has the power to repel whatever threatens him. Later, the light begins to appear when Lucia is in danger as well. In chapter 16, Kaito's light even surrounds Lucia and makes her immune to Maria's attack. However, Kaito's light is always accompanied by headaches, which become more and more paralyzing for Kaito, especially as his light increases in power.
healing
Kaito has the ability to heal his own injuries and illnesses much faster than a normal human could.
underwater breathing
There are instances in the manga when Kaito demonstrates the ability to breathe underwater, most notably during the end of the first story arc, and in chapter 26, when he makes out with Lucia underwater. In chapter 21, Sara tells Lucia, "Kaito cannot die within the ocean," since he is descended from the Panthalassa kings. When Kaito and Lucia first met, Lucia thought that she saved Kaito from drowning, but in chapter 32 of the manga, it is revealed that Kaito actually saved her. Still, the incident nearly killed Kaito, and Lucia still decided to give him her pearl in order to revive his strength anyway.
Magical Items
royal staff
Rihito inherits a regal, rose-topped rod from his mother, who is of Panthalassa blood. The rod very much resembles the one weilded by Gakuto in the first story arc of the manga. During the climax of the second story arc, Rihito gives the rod to Kaito, and Kaito uses the rod to smash the fossilized remains of the original Michel.
other weapons
In his "prince" form, Kaito has no actual weapons - although he does manage to valiently weild a convenient candelabra. In some illustrations he is drawn with a sword.
Aqua Regina is referred to by Nikora (and later, Fuku-chan) as "the Goddess of the Ocean World." Aqua Regina was once a powerful being, but by the time that the manga series begins, she is nothing more than a ghostly specter haunting the ocean. Why? The flickering, ghostly visage of Aqua Regina appears to Lucia in chapter 06, and explains that in the past, when the Panthalassa clan threatened the Earth, she used all of her power in order to seal them within the deep ocean. (In chapter 17, Gakuto himself confirms that this happened tens of thousands of years ago.) Aqua Regina depleted her power in doing so, however, which is why she now exists only as a ghostly spirit.
In order to fully revive Aqua Regina, the seven mermaid princesses must combine together the power of their seven pearls. And reviving Aqua Regina is the only way to stop Gakuto this time around. Gakuto further explains that once the seal upon the Panthalassa is broken once, it can never be repaired again without Aqua Regina's power. And the seal is broken, thanks to Sara.
Until the seven princesses could gather together again, the ghost of Aqua Regina remained protecting the kingdom of the North Pacific Ocean, the last remaining mermaid kingdom, from Gakuto's attacks. In chapter 20, when Aqua Regina is finally called forth by the mermaid princesses again, she repairs the seal on the Panthalassa clan, and sends Gakuto (and Sara, by her own choice) down to the bottom of the ocean. There, she says, "Gakuto's ocean will embrace them both in a peaceful sleep." After that, Aqua Regina vanishes.
In the second story arc of the manga, after Seira is born and the seven mermaid princesses can combine their song once again, Aqua Regina is, yet again, summoned. This time around, however, Aqua Regina does not play much of an active role in the final battle, save to give words of encouragement to the mermaid princesses. She may or may not be responsible for summoning the "ancient ones" that appear to save Michel - the manga leaves that bit unclear. However, it is Aqua Regina who performs the final miracle that returns baby Michal to Rihito.
