Definitely the followers of the Olympian gods who are extremely loyal and fear any connection to Kronos. They would consider Percy a walking danger because of his relation to Kronos. Another group could be some of the ancient monsters that were once under Kronos' control. They might sense Percy's connection to Kronos and either try to serve him (which Percy doesn't want) or try to destroy him to sever that link. And there could be some dark demigods who have been corrupted by the idea of Kronos' return and see Percy as a key to that return, but in a way that goes against Percy's own will, so they would try to manipulate him.
The Olympian gods could be the main antagonists. Since Percy is the adopted son of Kronos, they might see him as a potential threat and try to stop him before he can fulfill any of Kronos' possible plans. They may send their demigod heroes after him.
The Titans who oppose Kronos' idea of having Percy as his adopted son could be antagonists. They might view Percy as a weak link or a tool that Kronos is using wrongly. So, they would try to either kill Percy or turn him against Kronos. Also, some of the more power - hungry demigods at Camp Half - Blood who see Percy as a rival for power could also be antagonists. They could spread rumors about him and try to undermine his position among the other demigods.
The best chapter pointer here is Chapter 266, "He Really Deserves to Die!". It opens with Upon hearing this, the deputy director's expression also turned serious. and closes on Now I see why Jiang Li despises her brother so much, utterly unreliable!"....
Chapter 264 is the strongest chapter-level answer because it starts from Jiang Li first reassured her mother, then opened the chat box with Shen... and follows through on [Do your faces hurt, you rumormongers?.
Look to Chapter 276 for the clearest answer. It begins with After listening to Jiang Li's words, Shen Lan's brow furrowed again., keeps building through Jiang Li, I think I might know who this person is., and lands on Don't overthink it and get some rest." Jiang Li nodded..
Chapter 227 is the strongest chapter-level answer because it starts from Your Four Pillars of Destiny is very strong, with wealth, authority, and... and follows through on Your three palaces are prosperous, and correspondingly, your marriage....
A practical answer is Chapter 269, because it turns the idea into a concrete scene. The chapter opens with The next morning, Shen Lan's personal account posted a Weibo., adds pressure through Too excited to remember to switch to his alternate account, he..., and ends on At that time, people.
If you want the clearest chapter reference, go to Chapter 227. Early on, Your Four Pillars of Destiny is very strong, with wealth, authority, and...; later, the chapter leans on The comments section was filled with amazement as well..
A practical answer is Chapter 271, because it turns the idea into a concrete scene. The chapter opens with He had barely finished speaking when he received a withering look from..., adds pressure through Forget about your brother;
A practical answer is Chapter 250, because it turns the idea into a concrete scene. The chapter opens with The knowledge that several guests were soon returning had fans arriving at..., adds pressure through Not seeing me for so many days and not even asking how I am, you think—"..., and ends on.
A practical answer is Chapter 220, because it turns the idea into a concrete scene. The chapter opens with I know everyone has many questions, and I'm here to answer them one by..., adds pressure through Carrying the heat surge of his research topics, he instantly shot to..., and ends on However.
Look to Chapter 243 for the clearest answer. It begins with Hahaha, the young master's face has gone white, but I got a scare just now..., keeps building through Who knocks on the door in the middle of the night, especially in a place..., and lands on Perhaps feeling that the atmosphere had become.