I vaguely recall a series of episodes (perhaps not all consecutive?) in Quantum Leap that dealt with a "dark" (i.e. bad, evil) equivalent to Sam.
Which episodes of which seasons did this dark jumper appear in?
They were more commonly referred as the Evil Leapers, and they appeared in 3 episodes in the fifth season : "Deliver Us From Evil", "Return Of The Evil Leaper" and "Revenge Of The Evil Leaper". The first 2 episodes featured Alia as the leaper and Zoey as the hologram; in the third, Zoey is the leaper.