One potential new character could be a selkie. Selkies are mythological creatures that can transform between seal and human form. This selkie could be an ally or a foe to Percy. Maybe there's a sea nymph who has a special connection to Percy's mother. She could show up to help Percy when he's in a tight spot. And what about a sea giant who was once an enemy of Neptune but now wants to make amends? He could be a very interesting addition to the story.
In fan fiction for 'Percy Jackson the Son of Neptune', we could introduce a character like a water elemental. This being is made entirely of water and has its own consciousness. It could be either a friend or an enemy depending on Percy's actions. There could also be a young sailor who has a natural affinity for the sea and ends up being dragged into Percy's world. He has no demigod powers but is really brave and resourceful. And perhaps a sea witch who has her own motives and uses her magic to manipulate the events around Percy.
A mermaid - centaur hybrid could be a really cool new character. This character could have the upper body of a centaur and the lower body of a mermaid. They could be an expert in both land and sea magic, and help Percy on his quests. They might also have a mysterious past that is related to the ancient wars between the gods.
A character like a demigod with the power to control time could be really interesting. This character could be from a minor god's lineage. They could use their power to help Percy in battle, for example, by freezing time for the enemies while Percy attacks.
A character could be a young oracle who has visions related to Percy's new adventures. She could be a bit of an enigma, not fully understanding her own powers and sometimes giving cryptic messages that Percy has to decipher. This would add an element of mystery to the story.
There could be a character who is a mentor to Percy in understanding his devilish side. This character might be an ancient being with knowledge of both the divine and the demonic. For example, a fallen angel who has seen the rise and fall of many with such mixed heritages.
There could be a new oracle who is specifically attuned to Percy's 'son of fate' situation. This oracle might be a young demigod with unique powers that help Percy understand his fate better. Maybe they can see visions of the future that are directly related to Percy's journey.
Perhaps some minor gods who have been overlooked in the past. They could see Percy's new status as the 'Son of the Universe' as an opportunity to gain more influence and decide to ally with him. For example, the god of small rivers or the goddess of wildflowers.
A young demigod who has the power to communicate with the spirits of the monsters. This character could help Percy understand the motives of the monsters better and maybe find non - violent solutions to some of the conflicts.
The main character is Percy Jackson. Since he is the son of Neptune, he's at the center of the story. However, in the Percy Jackson universe, there are often other important characters. It's possible that Hazel Levesque, a character from Camp Jupiter, could be in it as she has her own connection to the Roman side of the mythology which ties in with Neptune. Also, Frank Zhang, another Camp Jupiter demigod, might be a part of the story. They could be Percy's allies or play important roles in the events that unfold in the graphic novel.
A centaur who has lived in both the Roman and Greek territories. He could be a mentor to Percy, teaching him about the similarities and differences between the two cultures' ways of combat, magic, and survival. He could also have a lot of stories about the ancient times when the two cultures were more intertwined.
Well, there could be new demigod children of Hades. Maybe a girl who has the power to control the shadows in a unique way, like she can use them to create portals to different parts of the underworld. Or a boy who can communicate with the dead in a more advanced way than the usual children of Hades, and he has some sort of prophecy related to Percy.