Other gods and goddesses could interact with Percy. For example, Athena, the goddess of wisdom, might offer her advice to Percy on how to better manage his creations. Or Poseidon, if Percy is not Poseidon himself in this fanfiction, could interact with Percy regarding the creation of sea - related creatures.
Mortals could also be main characters interacting with Percy. A young and curious mortal might stumble upon Percy's creation realm and Percy has to decide what to do with this mortal. Does he let the mortal stay and learn, or send them away? Another could be a powerful mortal king who asks Percy to create something specific for his kingdom, like a magical defense system.
There could be new characters as well, like a devil mentor figure who teaches Percy about his devil heritage. This character could be ancient and have a lot of knowledge about the devil realm. Or there could be an angelic character who is initially an enemy but later realizes Percy is not fully evil and they end up forming an unlikely alliance.
Obviously Naruto would be one of the main characters since it's about him as the god of creation. Sasuke could also be a main character. He might be Naruto's ally or rival in this new story, depending on how the fanfiction is written.
There could be other gods as well. For instance, Zeus might play a role, perhaps being suspicious of Percy's newfound godly powers or seeing him as a threat to his own authority. Hermes could also be there, acting as a messenger or a guide to Percy in the world of the gods.
The God of Creation himself could turn out to be the main antagonist. He might start off as a benevolent figure but then reveal his true, more self - serving motives. Maybe he wants to reshape Remnant in his own image, regardless of the cost to its inhabitants.
Probably other demigods would interact with Percy. Maybe some demigods who are in need of a more stable influence in their lives would seek out Percy. Also, characters like Grover might find Percy in Hestia's care and there would be a reunion. And the gods and goddesses who are associated with the concept of home or family in Greek mythology, like Demeter, could also have interactions with Percy.
Obviously, Percy is one of the main characters since the title implies it. He would be the central figure whose story the fanfiction revolves around.
The main character is Percy. However, in the fanfiction, there could be a new character created specifically for the story, like a young apprentice of Percy who has a unique ability. There may be an old and wise god who has some secrets about Percy's past and is reluctant to share them at first. Also, there could be a group of demigods who are initially skeptical of Percy but then come to believe in him and join his cause as he tries to reclaim his position among the gods.
Obviously, Percy would be the main character. As he is the one born a god in this fanfiction.
He would surely interact with Annabeth. Their relationship is central in the Percy Jackson series, so in a Tartarus fanfic, she'd likely be there to support him or be in a similar situation as him.
Obviously, Percy and Hades will interact a great deal. Percy will also likely interact with the Furies, as they are servants of Hades. They could be like his protectors or trainers in some sense. Another character could be Nico di Angelo, since he has a connection to Hades as well. They could bond over their shared relation to Hades.