Mister Hyde is the terrifying, borderline-demonic "split personality" of Doctor Jekyll, fought in the second half of the boss battle at the end of "Welcome to Millhaven".
Before entering the church, Rusty encounters a spirit who warns him that Jekyll is "half-creature", and should be defeated while he's still human.
Despite an apparent shared history between Jekyll and Rusty Nails, Rusty seems to be unfamiliar with "Mister Hyde" when Jekyll drops hints about him. After Jekyll falls in battle against Rusty, the interior of the Millhaven church takes on a morbid new appearance, and Mister Hyde rises from the spot where Jekyll's body previously fell.
Following Hyde's defeat, the Millhaven church collapses in on itself with a series of explosions that Michael Bay would be proud of, and the spirits of Millhaven's dead ascend to their long-awaited eternal rest.....
....but a last, lingering shot of the ruined church reveals that Hyde may not be as dead as Rusty thinks....
Connections to "The Strange Case of Dr. Jekyll and Mr. Hyde"Edit
Unlike most depictions of Mr. Hyde, the AMC TC version doesn't just make Hyde an uglier, more evil Jekyll---here, he becomes a full-on demon. It's not the first time Hyde has been depicted as being a super-powered brute, however; the VERY loose "adaptation" of The League of Extraordinary Gentlemen depicted him as a Hulk-like giant, a characterization that was echoed in the opening to Van Helsing.
Despite Jekyll being somewhat amoral in his own right (working with the cultists to slaughter the entire population of Millhaven, for instance), Hyde, in the AMC TC, is practically a beast. He retains none of Jekyll's humanity, and is seen clinging to the church belltower like King Kong after Rusty leaves Millhaven.