The antagonist of the book is Luke Castellan, a seemingly friendly half-blood camper who turns out to be the leader of a group of rebellious demigods called the Children of the Titans. Luke is working to free Kronos, the ancient Titan who was defeated by Zeus, but he doesn't actually embody Kronos himself.
Kronos is a powerful force who exists in the background of the story. He's mentioned by Luke as the one he's trying to free, and the god of war, Ares, even refers to Kronos as the "big bad" Luke is trying to release. However, Kronos is a shadowy figure who doesn't appear physically in the book.
The story focuses on Percy's journey to clear his name after being accused of stealing Zeus's lightning bolt, and his eventual discovery of the true mastermind behind the theft, Luke.
To learn more about Kronos, you'll need to read the later books in the Percy Jackson series, especially the "Heroes of Olympus" saga.