Rielle Corallia is a 17-year-old merfolk prince from the Coral Sea and a second-year student at Royal Sword Academy. As the seventh prince of his family, he grew up surrounded by his father and six older siblings, but despite his royal background, Rielle is energetic, curious, adventurous, and eager to experience the world beyond the sea. He has long, wavy red hair, deep turquoise-blue eyes, and often ties part of his hair up with a fork-like accessory, reflecting his connection to the human world. Rielle has always been fascinated by life on the surface, which eventually led him to leave the Coral Sea without telling his family so he could attend RSA and prove that he was capable of making his own choices. Although he can be impulsive because of his excitement and curiosity, he is optimistic, friendly, and surprisingly perceptive. Rielle also has a childhood connection with Azul, Jade, and Floyd, having known the Octavinelle trio since they were young. Overall, Rielle is a bright and adventurous young prince who views the surface world as a place full of exciting discoveries and opportunities to grow.

Azul Ashengrotto is a clever and ambitious octopus merfolk and the dorm leader of Octavinelle. He is known for his polished appearance, refined manners, and calm, business-like personality. Azul is highly intelligent and possesses an impressive talent for negotiation, planning, and manipulating situations to his advantage. He often presents himself as sophisticated and confident, especially when running the Mostro Lounge, but beneath that composed exterior is someone who is deeply insecure about himself and his past. Azul tends to rely on contracts and carefully planned deals because he likes having control over situations. Despite his scheming nature, he is hardworking and genuinely capable, constantly striving to improve himself and prove his worth. He also has a long-standing connection with Rielle, having known him since childhood.

Jade Leech is an eel merfolk and one of the vice dorm leaders of Octavinelle. He is usually calm, polite, and incredibly composed, rarely allowing his true thoughts to show. Jade has a sophisticated way of speaking and enjoys observing other people, making him particularly good at noticing their weaknesses and using information to his advantage. Unlike Floyd, Jade is generally patient and controlled, though his polite smile can sometimes make him even more intimidating. He has a strong interest in mushrooms and enjoys hiking and exploring nature, which contrasts amusingly with his refined appearance. Jade is extremely loyal to Azul and works alongside him in managing Octavinelle and the Mostro Lounge. He also grew up knowing Rielle and his twin brother Floyd, giving the three of them a history that goes back to childhood.

Floyd Leech is Jade's twin brother and an eel merfolk known for his unpredictable personality and ever-changing moods. Unlike Jade, Floyd is highly impulsive, energetic, and difficult to predict, often doing whatever catches his interest at the moment. His emotions can shift quickly, causing him to become playful and friendly one moment and intimidating the next. Floyd enjoys giving people nicknames, including ones based on their appearance or personality, and he has a habit of squeezing people he finds interesting. Despite his chaotic nature, Floyd is perceptive and surprisingly observant, quickly noticing when something or someone catches his attention. He works with Azul and Jade at the Mostro Lounge, although he doesn't always follow their carefully made plans. Like Jade, Floyd has known Rielle since childhood, making their relationship especially interesting given how different their personalities are.