Joe Ridley

Joe Ridley is the secondary antagonost of the X-Files episode "Young At Heart", unlike many of the show's villains Ridley is not an evil man and his goals were very much noble, however, his methods and means by which he sought to achieve these goals were at odds with ethics and made him a "mad scientist".

History
Joe Ridley was a scientist working on the rare disease known as progeria and became convinced he could cure the disease via slowing or even reversing the human ageing process - when his attempts to legally have human testing of his work failed he went rogue and began experimenting on outpatients.

This caused his fellow doctors to rebel against him, labelling him "Dr. Frankenstein" and "Dr. Mengele" due to his obsession with the ageing process and how he often treated his patients as lab rats rather than full human beings.

Unsurprising in 1979 Ridley's medical license was officially rebuked but he found work with the Syndicate, who were interested in his experiments and hoped to see results - since the idea of possible biological immortality obviously delighted the Syndicate and they did not care much for ethics.

Thuus Ridley would get his chance at true human experimentation and did so with the notorious bank robber and murderer, John Barnett.

Abilities
a brilliant but ethically void scientist of the highest order Ridley can be considered a genetic genius who managed to unlock the key to not only slowing the human ageing process but reversing it completely - thus he effectively made Barnett biologically immortal.