It could be an old enemy of Percy's who sees Leah's imprinting as a way to get to him. For example, a powerful Titan or monster might try to use Leah as a hostage or a tool to manipulate Percy.
In this fanfiction, the imprinting might also be a source of internal conflict for Leah. She could struggle with her newfound feelings and loyalty towards Percy while also having her own personal goals and identity. Maybe she has to make a choice between following Percy on his adventures and fulfilling her own responsibilities, and this internal struggle could drive the plot forward.
It could be a new vampire character created specifically for the fanfiction. Maybe a young and somewhat rebellious vampire who doesn't fully adhere to the traditional vampire ways.
Without more context from the specific fanfiction, it's hard to say exactly who Leah is. She could be an original character created by the fanfiction writer, perhaps someone with special powers or abilities.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.
Maybe it could be a demigod who has become obsessed with power through gaming. This demigod, let's say a child of Hades, discovers a way to use dark magic and combine it with gaming technology to try and rule over all the other demigods. He sees Percy as a major threat to his plans because Percy is so popular and has strong powers, so he constantly tries to undermine Percy at every turn.
The goddess Hera might also be a great antagonist. Since she has a history of interfering in the lives of demigods, she could be jealous or threatened by Percy's rebirth. She might try to stop him from reaching his full potential in his new life or cause trouble between him and his new allies.
A powerful demigod who has been corrupted by the idea of primordial power could be the main antagonist. They might believe that by allying with a primordial or obtaining their power, they can become greater than the gods. This demigod would then oppose Percy, who stands for the protection of the current order.
An ancient and forgotten god who has been awakened. This god could be angry at the modern demigods for ignoring the old ways and decides to send assassins after Percy as a warning to the rest. They might want to bring back the old order where gods were feared and worshiped more and see Percy as a symbol of the new, more independent demigods.