An antagonist is a character that causes challenges for the protagonist, or main character.
In To All the Boys I've Loved Before, Lara Jean is the protagonist, as she is the central character. This means that we must consider who "opposes" her most.
In a roundabout way, Kitty is an antagonist. Although Lara Jean doesn't know it until the end of the book, Kitty causes the main conflict by mailing Lara Jean's hatbox full of letters. This propels the entire plot and causes the most problems for Lara Jean. Kitty isn't exactly a typical antagonist, however, because she indirectly causes the various conflicts and is not consistently malicious towards Lara Jean.
Another option is Genevieve. Although she doesn't catalyze the entire conflict of the story, she directly opposes Lara Jean by spreading the rumor about her and Peter in the hot tub. This paints her as more of a classic antagonist, particularly in a book set in a high school.