As the source of her insanity and the former object of her affection, he’s a central part of her arc, even though the show will focus on Harley after leaving Mr. J. This version of the character sounds like he will be a villain who goes after Harley following their rocky breakup.
Right off the bat (pun intended), it’s worth saying that The Joker was a likely choice to include among the supporting cast of the Harley Quinn series, as the character is arguably the most iconic villain of all time — having been portrayed by icons like Mark Hamill, Jack Nicholson, and Heath Ledger.