Dr. Jekell is a doctor who helps you in the "A Huge Favor" questline.
Quest: Another FavorEdit
Dr. Jekell tells you that you must pay 50 million Coins in order to get a Space Stone, due to the Space Stone being WORTH 50 million Coins. When you pay 50 million Coins, he'll give you a Space Stone. However, this leads to Dr. Jekell mutating into Mr. Hiddyn. You must defeat Mr. Hiddyn before exiting the laboratory. But before you exit, Dr. Jekell tells you who can cast an Enhancement spell on the Space Stone.
Mr. Hiddyn - 100 million HP, level 120
About Dr. Jekell/Mr. HiddynEdit
Dr. Jekell and Mr. Hiddyn is, obviously, based on Dr. Jekyll and Mr. Hyde.