The King

The King is the primary "antagonist" in Mabel. He has no credited voice actor, but one can assume his voice is a corrupted version of Becca De La Rosa's.

All Seasons
The King rules the Underhill as all tyrants tend to-- like a business. He kidnaps Luna Thorne and cages her; eons later, he does the same to Mabel Martin. He is challenged by Anna Limón and loses his throne to her. It is currently unknown what he is up to.

He isn't the monster at the end of the book, either-- that's Mabel-- but something more complex and fickle than most human stories could ever nail down. He actually helps the Girls With Subzero Chill and is said to actually care about Mabel (in a non-romantic way, obviously). There are no canonical points as to his design.