William Heo is a man working inside Castle Curtis alongside (or at least being monitored by) Death, Amelia, Cipher and possibly others; it's implied he's working with Vlad just as he did hundreds of years ago, but as he has yet to appear, this remains speculative. He has a close relationship with Melanie who he treats as a granddaughter, and Alfred, his right-hand man. His name in Japanese is Judah (ユダ) and it is the death of his wife that motivates him. His Japanese character bio clarifies that he was given power and a "Steel Heart" (鋼鉄の心臓) by Vlad and assisted him in spreading the Plague across Europe in the 1300s. His age is given as 62 though he has been alive for much longer.
Continuing his plans from over 300 years ago, he sealed Thanatos in his stone form and stole a fragment of his heart in order to create the Other World, a limbo where souls gather and can be resurrected; in the D-4 Ending, he resurrects Pamela and is likely responsible for the resurrection of Lord Curtis as well. It stands to reason that he plans on using this power to restore his wife and cares little about what the others use this power for.
He is voiced by Unshō Ishizuka in the Japanese version of the game.