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.
Well, Hinata could be a main character. She might be in awe of Naruto's mermaid form and be very protective of him. Jiraiya could also play an important role. He might be intrigued by Naruto's new form and try to train Naruto to use his mermaid powers in combination with his ninja skills. And of course, there could be mermaid characters native to the underwater world that Naruto interacts with, like a mermaid princess who becomes Naruto's friend and helps him understand the ways of the underwater world.
In addition to Naruto, Kakashi could be a significant character. He could be the one guiding Naruto on how to balance his mermaid nature and his ninja duties. Also, there could be a villain character, perhaps a rogue ninja who wants to capture Naruto because of his unique mermaid abilities. This villain could add conflict to the story. And Tenten might also be involved, she could be the one who creates special weapons or tools for Naruto to use underwater, using her skills in weaponry.
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 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.
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.
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.
Sasuke could be one of the main characters. At first, he might be among those who doubted Naruto, but as Naruto starts to prove everyone wrong, Sasuke has to re - evaluate his views. This could lead to some interesting character development for Sasuke as he sees Naruto's growth and realizes his own misjudgment.
Sakura could be one of the main characters. Since Naruto is the last male, Sakura, being his friend, might play an important role in helping him deal with the situation. She could use her medical skills to support Naruto in his journey.
Kakashi could also be a main character. He could be the one who has to deal with Naruto's failure in a different way than Iruka. Maybe he has to decide if Naruto is worth another chance. Also, Sakura could be involved. She might initially be disappointed in Naruto but then realizes that there is more to Naruto's failure than meets the eye and tries to support him in her own way.
A mermaid named Nami could be one of the main characters. She might be a curious and adventurous mermaid who is the first to approach Naruto. Another could be Marina, a more reserved mermaid but with great wisdom. She could act as a sort of advisor to Naruto within the mermaid world.