Who is the antagonist in Midnighters: The Secret Hour by Scott Westerfield?i need the antagonist( could be a darkling.)

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.