April O'Neil is a classic choice. She's a human character who has a long - standing relationship with the turtles. Her skills as a reporter can add an interesting dynamic to the story.
A new human character that could be introduced is a scientist who is intrigued by the turtles' mutation. This character could either help them or cause problems as they try to study the turtles' unique physiology. They could bring in new scientific concepts and technologies to the story, making it more complex and engaging.
Leonardo is often popular. In human fanfiction, he might be depicted as a strong and noble leader even in human form, guiding the other turtles and any human allies. He could be the one who tries to uphold justice in a human world.
Leonardo is often very popular. He's the leader, so fanfictions might focus on his leadership challenges and how he makes decisions for the team.
Casey Jones. He's often overshadowed by the turtles. But in fanfiction, his brash and unpredictable nature could be used to create some really interesting storylines. He could have his own solo adventures that tie into the turtles' world in unexpected ways.
One idea could be a story where the turtles become human for a day and have to navigate high school. They'd struggle with normal teenage things like fitting in and dealing with bullies, all while trying to hide their turtle identities.
One idea could be a story where the Teenage Mutant Ninja Turtles discover a hidden mecha technology in the sewers. They have to figure out how to use it to defeat a new, powerful enemy. Maybe it gives them new abilities like enhanced speed or the power to fly.
Perhaps a new villain emerges. A powerful sorcerer who is angry that the turtles have been turned human by a magic that he believes should be under his control. So he tries to either turn them back into turtles in a more twisted way or simply destroy them in their human forms because he sees them as abominations.
There could also be a storyline where a female character is from a different dimension. She gets stranded in the turtles' world and has to find a way back home. Along the way, she forms a special bond with the turtles, especially one of them, and they all go on a journey to find the key to send her back. It's a story full of friendship and exploration.
One idea could be a story where the turtles have to blend in as humans in a high school setting. They could struggle with normal teenage things like exams and dating while also hiding their turtle identities.
Another great graphic novel is the classic 'Teenage Mutant Ninja Turtles: Mirage Comics'. It was the origin of the turtles in the graphic novel world, with a unique art style and engaging storylines that introduced the characters like Leonardo, Donatello, Michelangelo, and Raphael to fans in a new way.
The main characters are the four Teenage Mutant Ninja Turtles: Leonardo, Donatello, Raphael, and Michelangelo. And of course, Splinter and April O'Neil.