Sakura could be a main character. Since it's Naruto returning to Konoha, Sakura, being his friend and a strong ninja in her own right, might play a big role. Maybe she's the one who first welcomes Naruto back and they team up to face any threats.
Well, it could be about Naruto coming back to Konoha with new powers. Maybe he's been on a long journey and is now ready to share his experiences and help protect the village again.
Sasuke could be one of the main characters. He might be initially skeptical or even hostile towards Naruto's mermaid side, but as the story unfolds, he could come to understand and support Naruto. Another character could be Sakura. She could use her medical ninjutsu skills to help Naruto if he gets injured during his underwater adventures.
Samul, of course, since he's Naruto's supposed father in this fanfic. He could be a powerful ninja who had some great influence in the past. Another character could be Naruto's mother, who might have a different story now considering her relationship with Samul. Also, characters like Jiraiya could still play an important role. He might be the one to guide Naruto in understanding his new identity as Samul's son and help him train to face the challenges that come with it.
Hinata is also a likely main character. In the original story, Hinata has a crush on Naruto. With Naruto being a smooth operator, he might notice Hinata's feelings more easily and their relationship could develop in a more romantic and interesting way. Kakashi could also be involved. Naruto could use his smoothness to learn new jutsu from Kakashi more effectively or to get Kakashi to trust him with more important missions.
The main characters are, of course, the Konoha Twelve. This includes Naruto Uzumaki, Sasuke Uchiha, Sakura Haruno, and others. They are the focus of the fanfic and usually the ones driving the story forward with their actions and decisions.
Another main character could be Kakashi. As Naruto's former teacher, he might be conflicted about Naruto's exile. He could be trying to find a way to bring Naruto back to Konoha or at least help him in some way while also having to deal with the village's decision. And of course, there could be new characters introduced specifically for this fanfic, like a mysterious figure who helps Naruto during his exile.
The main characters would most likely start with Naruto. His character would be central to the story. Alongside him, we might see the Hokage at the time also playing a role. Additionally, there could be a group of shinobi who are specialized in dealing with demons that Naruto teams up with. And perhaps some of the villains or demons themselves could be considered main characters in a sense, as they drive the conflict in the story.
The street kids themselves would be main characters. They could be a diverse group with different personalities and backstories. There might also be Naruto's friends like Sakura and Kakashi who could play important roles. Sakura could help with the kids' medical needs, and Kakashi could offer his wisdom in training the kids.
Sakura could be a main character. She might be the one who takes on a motherly role towards baby Naruto. She'd be very protective and try to teach him basic things. Kakashi could also be important. He'd use his wisdom to figure out how to deal with this situation and also keep an eye on Naruto's safety.
Urahara Kisuke would be a great addition. His mysterious nature and vast knowledge could either help or hinder Naruto. Yoruichi could also be a main character. Her cat - like abilities and combat skills would make for some exciting interactions with Naruto. She could teach Naruto some new moves or they could team up against a common enemy.