The antagonist is not the main character. That is the protagonist. The protagonist is the hero, the antagonist is the villain. The darklings and slithers are antagonistic because they interfere with Jessica, the protagonist. Anyone who interferes with the protagonist can be an antagonist, but THE antagonist is the one villain against whom the character must fight. In a series like this one there is likely to be one super villain who is somehow responsible for some tragedy that it will be Jessica's responsibility as the hero or protagonist to stop. She will need to bring him to justice. So far, we only know that he is dark, secret and powerful. Now that Jessica has her power, she is going to end fighting him.