One character could be Jiraiya. He could have known something about Naruto's true heritage all along and be the one to finally reveal it to Naruto. From the Greek side, Triton, Poseidon's son in mythology, could be a character. He could either be a rival or an ally to Naruto. In the ninja world, Shikamaru might be involved. His intelligence could be used to help Naruto understand and strategize with his new powers, especially when dealing with any threats that come his way because of his connection to Poseidon.
Well, all the regular Naruto characters would still be there. Sasuke could be a major character. He might be initially skeptical or even jealous of Naruto's new identity. Kakashi could also play a role, perhaps trying to help Naruto control his new powers. And from the Shiva side of things, Nandi, Shiva's bull, could be a character. He could act as a protector or a guide to Naruto, teaching him about Shiva's ways and powers.
There could be characters related to Luna. Maybe Luna has some friends or family members who play important roles. Also, the regular Naruto characters like Sasuke and Sakura would still be there, but their relationships with Naruto might change due to his new identity as Luna's son.
Kakashi could also play a major role. He has a wealth of knowledge and experience, and could guide the son in his training. And then there's Tsunade. As the Hokage, she might be involved in decisions regarding the son's presence in the village and could also offer her medical expertise when the son gets injured during his adventures. There could also be some of the tailed beasts as characters, as they have their own powers and could interact with the son in various ways.
Perhaps there could be a storm - wizard or a shaman who adopts Naruto. This character would have deep knowledge of storm - related powers and would be the one to guide Naruto in his new journey.
Sasuke could be one of the main characters. He might be the one who comes across Naruto in his boxers and has a very typical Sasuke - like reaction, which could be a mix of annoyance and amusement. His presence could add some tension as well as some banter to the story.
Well, it could be about Naruto having special powers related to Poseidon. Maybe he can control water in ways that are different from his normal ninja skills. He might also have a connection to the sea or underwater realms.
Perhaps there is a crossover event in the fanfiction. Some sort of magic or powerful artifact causes the Naruto universe and the world of Poseidon to collide. Naruto, being the adventurous and powerful character he is, gets involved in this chaos and ends up having a special relationship with Poseidon, either as an ally or having to face him as an adversary.
Princess Luna herself would be an important character. Also, characters like Sasuke could be there, and their relationship with Naruto might change because of his new heritage.
Obviously, there would be the queen vampire, his mother. She could be a powerful and mysterious figure, perhaps with her own motives and plans for Naruto. There might also be some vampire elders or nobles who play a role in Naruto's life, either guiding him or trying to use him for their own purposes.
Sakura could be a main character. She might be the one who helps Naruto deal with his emotional and physical wounds after the war. She could also be struggling with her own feelings towards Naruto now that he's back.