A mermaid could be added. She could be an old acquaintance of the creature from the deep parts of the lagoon. She might have a different perspective on the humans and could either help or hinder the creature's interactions with them.
The main characters could be the Creature itself and a female biologist. The biologist is there to study the ecosystem of the lagoon and she accidentally crosses paths with the Creature. Their interactions then form the basis of the love story.
Typically, there are explorers or scientists. They are the ones who venture into the area of the Black Lagoon and encounter the creature.
In many such novels, there are often characters like a brave adventurer who is drawn to the mystery of the black lagoon. There might also be a local guide who knows a bit about the area but not the full extent of the creature's existence. And, of course, there could be a scientific researcher who wants to study the creature, thinking it could be a new discovery in the field of biology or zoology.
The main characters include the group of scientists. They are the ones who start this adventure in the Black Lagoon. There's usually a lead scientist who is in charge of the expedition. And, of course, the most prominent character apart from them is the Creature from the Black Lagoon. It's this mysterious and menacing being that the whole story revolves around. The creature has its own motives, which are mostly about protecting its territory and being curious about the humans, especially the female ones.
The unique features include its distinct art style which vividly brings the lagoon and the creature to life. The colors used often create a moody and mysterious atmosphere.
The main characters are likely to include the creature itself, and probably the humans who encounter it, such as the scientists or explorers.
How about a magical mermaid who lives in a hidden lake in the world of Sonic and the Black Knight? She could have the power to heal Sonic when he's injured and give him clues about the secrets of the Black Knight's sword, but she also has her own motives and secrets that Sonic has to figure out.
Typically, there are the scientists who are part of the expedition. And of course, the Creature itself is a main 'character' in a sense.
An old fisherman who has fished near the lagoon for decades. He has seen strange things over the years and when the Creature starts to make more appearances, he becomes involved. He could be a main character as he tries to protect his fishing grounds and deal with the Creature in his own way.
One common theme is isolation. The creature is often portrayed as being alone in the lagoon, cut off from the rest of the world. Another theme is the misunderstood nature of the creature. Fanfiction often tries to show that it's not just a violent monster but has emotions and a complex inner life.