Ygor
An enemy of the Frankenstein family, Ygor was tried and convicted for grave-robbing by a jury of eight men. He was sentenced to die by hanging. He survived with a broken neck, and because the sentence had been carried out, he was let go. This wasn't enough for Ygor, who wanted revenge on the eight men who'd crippled him. To this end, he befriended the Frankenstein monster, thought dead in the destruction of Henry Frankenstein's laboratory.
Teaching the brain-damaged monster to obey his commands, Ygor used him as his own personal hitman of sorts, sending him to murder the members of the jury who'd condemned him. The monster had already managed to slay six of the eight men by the time Wolf Frankenstein arrived to claim his heritage as the new Baron Frankenstein. This was lucky for Ygor, as the monster had become weak following a blow to the head. The sly old cripple managed to persuade Wolf to give the Monster the revitalizing electricity he needed.
As soon as the monster was well again, Ygor commanded him to murder the remaining two members of the jury, Edwald Neumuller and Emil Lang, and, just for good measure, Wolf's talkative butler Thomas Benson. However, killing Benson almost spelled the end for Ygor, as, in vengeance, Wolf shot him with a revolver and he seemed to die. He survived, but was crippled further. After surviving the sulphur pit, the monster became increasingly weaker, and thus Ygor took him to the town of Visaria.
In Visaria, Ygor blackmailed Wolf's brother Ludwig Frankenstein into treating the monster's brain damage. When Ludwig revealed he intended to give the monster a whole new brain altogether, the sly old cripple, desiring a new and stronger body, secretly persuaded Ludwig's fame-hungry colleague Dr. Bohmer to put his brain into the monster's head (instead of that of Dr. Kettering). Bohmer did so, but due to differing blood-types, Ygor's brain began to decay. After killing Bohmer in a rage, the Ygor-Monster destroyed Ludwig's laboratory and seemingly perished in the ensuing fire.
Although the Frankenstein monster lived on, Ygor's mind did not. It degenerated into an even more sub-human intellect than that of the monster's original criminal brain.
