Alberto lived as an orphan on a small island containing an abandoned lighthouse after his father left him. He made the lighthouse his own and lived by himself, collecting objects owned and created by humans; he also left tally marks on the wall, one for each day he lived without his dad. Even before meeting Luca, who later became his best friend, he carried an interest in such items from where the humans lived and stored them in the lighthouse. At the time of his first speaking appearance, he is still orphaned at the age of 14 years old. In Alberto's human form, he is a tall, tan-skinned boy with a slight suntan and curly light brown hair that is styled upwards. He also has green eyes and freckles. He wears a light yellow tank top, brown shorts that are cuffed at the thighs and a belt made of rope. Like Luca, he is always barefoot. In Alberto's sea-monster form (that only happens when he is in water or is wet), he has periwinkle scales, purple fin-shaped “hair” and green eyes with oval pupils and yellow scleras. He has a long tail with a thin, pointed end and purple caudal fins on his tail, forearms and calves. He wears the same outfit as in his human form. Alberto is presented as chill, but also inconsiderate. He also tries his best to seem cool and wise about the human world, something in which he prides in despite knowing nothing about it, and is shown to be risky and kind of a daredevil, having him perform actions that lead him to danger. Alberto is not one to take things seriously and is all about having fun, and is also admirable to certain objects or people.

Luca Paguro is a bright and inventive 13-year-old sea monster with endless curiosity – especially when it comes to the mysterious world above the sea. Although he's been warned his whole life that the human world is a dangerous place, he longs for something beyond his quiet farm life where he herds goatfish day after day. So when another sea monster with actual experience above the surface takes Luca under his fin, his eyes open up to a whole world of possibilities. In human form, Luca is a slender pre teen with light tan skin, rosy cheeks, dark brown eyes, and wavy, dark brown hair. He wears a teal striped ivory shirt, blue shorts, and is barefoot all the time. In sea monster form, Luca has blue-green scales and blue "hair". His eyes are still brown, but they have yellow sclera and oval shaped pupils. He also has a long tail with blue caudal fins. Before Luca obtains his new outfits, he is seen wearing green pants made of seaweed. Luca starts out as a very timid and shy character. He is afraid of the surface, but is also very curious about it. Luca is also a very kind-hearted individual who willing to stand up towards anyone who threatened to harm his friends.

Giulia did not believe in sea monsters and was frightened when Alberto revealed himself as one. She later discovered that Luca was one too, but knew they meant no harm later on. Giulia is a young Italian girl with red, short curly hair, brown eyes, and lightly tanned skin with freckles. She wears a sky-blue beanie, an orange-and-white striped T-shirt, turquoise jeans with a blue patch on the right knee, and brown sandals. Giulia Marcovaldo is an outgoing and charming adventurer with a love of books and learning. She’s an energetic and funny individual, but isn’t afraid to tell someone straight up.

Ercole Visconti is the alleged bully of the small Italian town of Portorosso and repeat champion of the town's Portorosso Cup race, despite how many people point out he is too old for it. He's a Vespa-owning, pompadoured blowhard who believes that everyone loves him and enjoys watching him eat sandwiches. He has two worshipful followers, Ciccio and Guido, who accompany him everywhere, ready to do his bidding. Ercole is a narcissistic bully, believing himself to be the best person in Portorosso and bullying others as a way to brag about his self-proclaimed success and as a show of strength. He cares deeply for his reputation, going to any length to keep his track record as winner of the Portorosso Cup. His ego causes him to delusionally believe that everyone in Portorosso adores and respects him when in fact nearly everyone despises him. Ercole has a slender build, a straight-laced face, lightly tanned skin, hazel eyes, dark brown slicked-back hair, and a few pencil-thin mustache hairs. He wears a dark blue polo shirt with thin, light blue stripes, periwinkle shorts, brown shoes, and a hot pink wool sweater wrapped around his shoulders. During the race, Ercole wears a red polo shirt with a thick white stripe in the middle and two white stripes on both sides. He also wears dark blue shorts, white fingerless gloves, and white socks. He still wears his brown shoes.

Massimo Marcovaldo, Giulia's dad, is an imposing, tattooed, one-armed fisherman of few words, killing anything that swims. Luca and Alberto can't help but be intimidated by his formidable size and skill with a knife, but Massimo has a soft heart, especially for his daughter. Massimo is a bulky adult Italian man with tan skin, short brown hair, bushy eyebrows that cover his eyes, and a short mustache that covers his mouth. He was born without his right arm. He wears a light yellow short-sleeved sweater and brown pants.