Doctor Jekyll, or just "Jekyll", is an antagonist of Rusty Nails fought in the first phase of a two-part boss battle at the end of "Welcome to Millhaven". Very little is known about Jekyll, other than some minor hints.
Jekyll is mentioned as being responsible for the slaughter that took place in Millhaven, and personally confronts Rusty in the town library; the two allude to knowing each other in years past, but little is confirmed. The remainder of the level centers around Rusty attempting to get to Jekyll, fighting through cultists and supernaturally-summoned creatures before their confrontation in the church.
Jekyll's appearance is that of a soldier wearing an unidentified uniform---the grey shirt, pants and the style of cap he wears suggest a soldier from the American Civil War, likely from the Confederate army. It's mentioned at least once that Jekyll may be the same Jekyll from Victorian England (see below for more), and one particular portion of Millhaven seems to connect him with the Phantasm film franchise.
After his defeat, Jekyll transforms into the terrifying Mr. Hyde.
Connections to "The Strange Case of Dr. Jekyll and Mr. Hyde"Edit
Obviously, Jekyll's name itself is a reference to the classic tale of man's constant battle with his own nature---but apart from that, this version of Jekyll is surprisingly less benevolent. Henry Jekyll has, in the majority of media depictions, been seen as
The "classic" Jekyll was never depicted as a soldier of any type, whereas this Jekyll seems to be wearing an American Civil War---another departure from the "classic" depiction. One intriguing facet of the Jekyll/Hyde character kept from most portrayals is that Jekyll's final "death"---in the novel, via suicide as Hyde; in the AMC TC, via being given the Boot Hill treatment by Rusty---leads him to transform into Mr. Hyde, though his transformation in this case changes the interior of the church with him.