Character Information: Minor Characters, Second Story Arc
The following are listed in order of appearance.
Manager-san | 
Rihito's manager is a patient, but not infinitely patient, man. He is constantly chaperoning around Rihito and his sister, and generally is very protective of their interests. He is also very kind - for example, he personally drives Lucia, Hanon, and Rina to one of Rihito's concerts. Above all, Manager-san is loyal to Rihito; he considers Rihito a musical genius unparalleled anywhere in the world. He only addresses Rihito, and refers to him, as "Maestro."
Interestingly, poor Manager-san's sanity almost cracks when Hippo, forgetting that he is still in his penguin form, casually strikes up a conversation with the poor man (in chapter 25).
Name Notes: In Hanamori Pink's introductory notes to chapter 25, published in volume 05 of the manga, she states that the manager's name is Makabe. Apparently this name was decided by one of Hanamori's assistants. However, the name Makabe is never mentioned within the actual story of the manga.
Rihito's Father | 
Rihito's father was a Japanese scientist who was driven to prove the existence of the ancient races (including the Panthalassa, mermaids, and the angelic race that Michel belongs to), no matter what the cost. The cost, in the end, ended up being his very soul.
In chapter 29 of the manga, Rihito's father confesses everything to Rihito. "Your mother... My wife, she was of the Panthalassa clan. Her health was weak from the beginning, but ultimately she left life in the water, in order to give birth to you and Michal. None of my fellow scientists would listen to me, but I had finally found proof that the ancient races existed. I had that proof in my own hands - my own children. Your Panthalassa powers should have manifested as you grew. But you, Rihito, your power only shined when you created music. And as for Michal, her body rejected her Panthalassa blood, and threatened her life the same way it did her mother's. Yes, that Panthalassa race's incredible power only brought misfortune, which is why so few of the Panthalassa survive today. Many of them must have met the same fate as your mother. With my research, I should have found fame and a respected position in academia, but... But from the relics that your mother left behind, I discovered the existence of another of the ancient races, one even more powerful than the Panthalassa. And I discovered this fossil [Michel] beneath the ice of Antarctica. At that time, there was that bird-creature [Fuku] there, and I sealed a contract with it. Yes... By injecting the DNA from that fossil into Michal's body, Michal would be infused with the power of the ancient races, and would be reborn."
Only it didn't work that way. Rihito's father was deceived by Fuku; what he created through his experiments on his daughter, Michal, was only a parasitic "fake" version of Michel. Rihito's father realizes his mistake too late. "I was so confident. I tested that DNA on my own body. Now I can no longer embrace Michal... I made a terrible mistake, and I even lost my own body..." Decieved by Fuku-chan, Rihito's father was transformed into the pillar of fire that guards Michel's castle.
In the end, however, Rihito's father is able to redeem himself. He sacrifices the last of his strength to help Lucia and Kaito break the fossilized remains of the original Michel, which destroys the foundation of the fake-Michel's castle, causing it to crumble, destroying the "kingdom" he had covered the Earth with.
Rihito's Mother | 
Rihito's mother was one of the Panthalassa clan. The manga does not detail how she met Rihito's father, but we do know that her health was poor from the beginning, and that living with Rihito's father on dry land only weakened her more. She died shortly after giving birth to her second child, Michal.
Rihito's mother inherited a powerful staff from the Panthalassa family. She entrusted the staff to Rihito before she died. Through the staff, she is able to appear before Rihito as a spirit, and ask him forgiveness for her sins, whether supposed or actual. Rihito's mother only appears before him once as a spirit, and then never again.
The Real Michel | 
The original Michel was the king of the ancient, angelic race that were the original inhabitants of Earth. But Michel was dismayed by the extinction of his own race, and spent the last of his energy praying to save them. Michel eventually "died," but the ancients preserved his body as a fossil, stored beneath the ice in Antarctica. There Michel and his servant Fuku slept for millions of years, waiting for their moment of revival.
The Michel that the mermaid princesses meet is not the original Michel, but rather a "fake," a clone created when the original Michel's fossilized DNA was injected into the body of a young girl with Panthalassa blood - one Amagi Michal.
The Ancient Ones | 
The ancient ones were the original inhabitants of Earth. However, as time wore on, the planet evolved to support human life, and the ancient ones realized that they were fading away. They wanted to prevent their own inevitable extinction. To that end, they concocted a desperate plan that ended up sacrificing their king, Michel, and his servant Fuku.
I've written a rather lengthy spoiler about the ancient ones and what they confess to the mermaid princesses in chapter 30 of the manga. You can read it here, in the spoilers section for Michel.