Character Information: Minor Characters, First Story Arc
The following are listed in order of appearance.
Momo-chan | 
Momo-chan is a baby dolphin who becomes stranded in the Momoha river, but is taken in by wildlife experts and eventually moved to a public aquarium. After Lucia and the other mermaids help reunite Momo-chan with his mother, Momo-chan returns to the ocean once again. Afterwards, he often acts as a messenger between the mermaid kingdom of the North Pacific Ocean and the mermaids living on land.
Momo-chan appears several times in the course of the manga. He can communicate his thoughts directly to Lucia via some sort of telepathy. But Momo-chan is also smart enough to be able to communicate on a basic level, through various means, with a thick-skulled human like Kaito as well.
Madame Taki
Madame Taki is a fairly major character in the anime, who only appears in the manga during a brief cameo in chapter 06 (literally, in only two panels), and then again for a larger role in chapter 32.
In the manga, Madame Taki is a fortune-teller who lives within Lucia's undersea kingdom. However, she is not a mermaid. But she is some sort of sea-creature, despite the fact that she looks very much like a human woman. We have no idea how old Madame Taki is, or how long she has lived. However, we do know that she has been present at the birth of so many mermaid princesses of the North Atlantic kingdom that she can hardly count them anymore. It is Madame Taki's role to view the future of each mermaid princess as she is born. Madame Taki also appears to have been responsible for Lucia's upbringing when she was a baby mermaid.
Meru | 
Meru is a mermaid from Hanon's kingdom, who managed to survive the devastation of Gakuto's attack. However, during the attack, Meru was separated from her mother. All alone, Meru managed to find her way to dry land, and eventually she also found Princess Hanon. Meru idolizes Hanon, but at the same time, Meru is not unwilling to listen to Yuuri's words as Yuuri questions why Hanon would leave behind her own ruined kingdom... After all, isn't that some sort of betrayal?
Meru appears to be just a young child, but she acts very adult-like for her age. Although at the same time she can act very immature and bratty, especially when around Lucia. Meru develops an instant crush on Kaito the moment that she meets him, further straining her abrasive relationship with Lucia.
Like other mermaids, Meru has her own pearl, and the ability to transform into a human. In her mermaid form, Meru's hair and tail are the same color as Hanon's hair and tail.
Name Notes: Meru's name is written with the characters for "flower bud" and "wandering, roaming."
Kura-chan | 
Yes, that's Kura-chan, sitting on Hanon's shoulder. This little jellyfish, a fairly important mascot in the anime, makes only ONE cameo appearance in the manga, in one single panel, on one single page. (Volume 2, chapter 07, page 70, in case you're wondering.) Oh, and eventually, Kura-chan also appears in the frontpiece illustration for chapter 30, but that's it.
The entirety of Kura-chan's dialogue? "I'm Kura-chan!" Thank you, talking jellyfish.
Nursemaid (Baaya)
This old woman is Sara's loyal servant, even after Sara turns evil. When Sara was younger, she called the old woman "Baaya" ("nursemaid"), but now, she doesn't bother to address the old woman at all. In the manga, the lower half of the old woman's body is never shown, so I couldn't tell you if there's supposed to be a fish tail or legs beneath her dress.
The old woman fears what Sara has become, and fears Gakuto even more, but barely has the power to speak out against either of them. She does, however, discretely betray her mistress twice, in order to help out Hanon. Not to mention the fact that this old woman is probably the one responsible for giving Princess Hanon something to tie around her chest, after Izuuru's fish knocks off her seashell brassiere (in chapter 14).
Johnny Kawamoto | 
Johnny is a rather full-of-himself surfer, the national champion of surfing in the year before Kaito competes in the championship. Johnny is fairly obviously used to getting any girl that he sets his eye on, and he immediately latches on to Lucia the moment that he first spies her on the beach. Johnny encourages Lucia to enter the Miss Mermaid contest, since the winner of that contest gets to give a kiss to the surfing champion. Lucia does enter, of course... But only because she can't stand the thought of another girl giving a kiss to Kaito.
Johnny may be arrogant and a bit too self-confident, but he is also a good sport. He is horrified when a slip of his hands results in Kaito becoming injured, and he pleads with Kaito not to compete in the final trials with a bad foot. "If Kaito has an accident in the final round," Johnny explains to Lucia, "he might not be able to surf anymore at all--!" Ultimately, however, Johnny backs down in the face of Kaito's determination and courage. And Kaito wins the championship, of course.
Kaito's Parents
Kaito's parents only appear in flashback sequences. They both died when Kaito was seven years old. While they were on a cruise ship with Kaito, a terrible storm destroyed their ship and caused them both to drown. Kaito's life, however, was saved by a mermaid...
Kaito's parents were both famous musicians. In particular, Kaito's mother was a famous organist. Kaito's father was a well-known composer, and a professor of music at an unnamed university. One of his students was once Mistuki Tarou. It was Kaito's father who also gave Kaito his first surfboard, as a present.