The character of Percy's younger brother in Zeus fanfiction could bring new dynamics. He could be a source of conflict, perhaps being jealous of Percy's achievements and trying to outdo him in the world of the fanfiction. Or he could be a support, working alongside Percy against common enemies. It all depends on how the author wants to develop the character.
If we consider Percy's younger brother in Zeus fanfiction, his backstory would be crucial. He might have grown up in Percy's shadow, which could fuel his actions. He could also have a special connection to Zeus that Percy doesn't. This connection could be used to drive the plot forward, like getting involved in a prophecy related to Zeus that Percy is initially left out of.
They could interact in many ways. For instance, Percy's younger brother might be a devotee of Zeus and constantly seek his guidance. Zeus, in turn, could use the younger brother for his own purposes, like having him perform tasks that Percy would refuse.
In this fanfiction, a great plot twist could be related to Percy's origin. What if Percy wasn't really born in the normal way as a god's offspring? He could be a product of some ancient magic that Zeus unknowingly participated in. This revelation could shake the very foundation of the relationship between Percy and Zeus. Also, Percy might discover a hidden side of Zeus' past that Zeus has been trying to keep secret. This knowledge could put Percy in a dangerous position as he decides what to do with this information, whether to expose Zeus or keep it to himself for the sake of their relationship.
The other gods would probably be very cautious around Percy. Since he's Zeus' younger brother, they would fear that any wrong move towards Percy could anger Zeus. For example, Poseidon might be more respectful towards Percy than he would be to an ordinary demigod, as he wouldn't want to create a rift with Zeus over Percy.
One possible plot could be Percy, as the brother of Zeus, having to deal with the politics of Olympus. He might struggle to find his place among the other gods while also trying to use his powers for good. Maybe he teams up with some of the younger demigods to bring about change in the rather old - fashioned ways of the Olympian council.
Well, one possible plot could be Percy discovering he's Zeus' brother in a very unexpected way. Maybe through some ancient prophecy that was long forgotten. He'd then have to deal with the power struggles within the Olympian family, as well as his own identity crisis. Another aspect could be his relationships with other demigods changing, as being related to Zeus is a huge deal.
The main antagonists could be some of the more traditional and power - hungry gods. For example, Hera might see Percy as a threat to the order she has maintained for so long. She could try to undermine him at every turn, fearing that his new ideas and his relation to Zeus will disrupt her plans. Another antagonist could be Ares, who might be jealous of Percy's potential power and influence over the other gods and demigods.
A fanfic about Percy, brother of Zeus, could start with Percy feeling a sense of isolation. Despite being part of the divine family, he doesn't quite fit in. He decides to leave Olympus for a while and explore the mortal world. There, he experiences the joys and sorrows of humanity in a way that Zeus never has. He falls in love with a mortal, which causes a huge rift between him and Zeus. This love story could be the central theme of the fanfic, with Percy having to choose between his family and his new - found love.
Fanfiction often takes creative liberties. In this case, it might be that the author wanted to explore new power dynamics and character interactions. If Percy was Zeus' brother, it would completely change the power balance in the world of the series. Percy's connection to the sea and Zeus' control over the sky could lead to all sorts of interesting plotlines, like joint quests or power struggles between the two 'brothers'.
To write a compelling Percy, give him flaws. Maybe he's overly impulsive because of his divine heritage, which often gets him into trouble. Also, show his internal struggle with being the son of Zeus. He could be constantly torn between living up to his father's expectations and forging his own path.
The younger brother's character can be developed by showing his struggle with his heretic nature. He could be conflicted between his vampire side and his witch side, which are both part of being a heretic. This internal conflict can make him a more complex and interesting character.