Kakashi could also be a main character. He has lost many people close to him, like his father, Obito, and Rin. Facing the death goddess, he could be in a moral dilemma. He might be tempted to use her power to change the past, but also realizes the consequences that could come with meddling with death. His internal struggle could be a central part of the fanfiction.
It could be Sakura. In the original Naruto series, she had feelings for Sasuke for a long time. But in this fanfiction, she could shift her attention to Naruto and be the one who wants him to be hers. Maybe after seeing Naruto's selfless acts and his growth, she realizes that he's the one she should be with.
Sakura could be a main character. She is a skilled medic - ninja. So, she might be involved in trying to figure out why the flowers are wilting. Maybe she thinks it's related to some new disease in the plant life of the Naruto world and she uses her medical knowledge to investigate.
Sakura could be a great main character. You could show her growth during the bell test, not just as a ninja in terms of skills, but also in her self - confidence. Maybe she has to take the lead when Naruto and Sasuke are incapacitated for some reason.
It could be a completely original goddess created by the fanfiction writer. This goddess might have unique powers like the ability to control the elements in a way different from the ninja in Naruto's world. She could be the goddess of light, for example, with the power to banish darkness and heal the sick.
It could be Sasuke. Since they have a complex relationship in the original series, in the yaoi fanfiction, their relationship could take a different turn in the Forest of Death. They might have more intimate moments while facing the challenges there.
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.
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.
It could be a completely original goddess created just for this fanfiction. Maybe she is the goddess of a forgotten element, like the goddess of lightening that has been long forgotten in the ninja world, but still holds great power.
Sasuke could be a great choice. He has that brooding, anti - hero vibe that could fit well in the criminal underworld. Maybe he gets drawn into the crime boss's schemes because he thinks it's a way to gain more power to achieve his own goals.