Ygor

Ygor as portrayed by Bela Lugosi in Rowland V. Lee 's 1939 Universal Feature "Son of Frankenstein."

The following is taken from Frankenstein Castle.

Like Frankenstein's creation, the broken-necked Ygor, a kind of monster himself, is a society outcast because of his crimes and his ugliness. Officially declared dead after being hanged, Ygor now haunts Frankenstein castle and is feared by children. To the villagers he has become a kind of ghost when he miraculously escaped the grip of death.

For Wolf Frankenstein (portrayed by Basil Rathbone) Ygor is like a Faustian Mephistopheles. He leads him to the Monster and convinces him to continue his father's experiments and to bring the creature back to life. But unlike Pretorius in Bride of Frankenstein, Ygor is not interested in science; his only goal is to finish his bloody revenge on the villagers. This makes Ygor the true evil character in Son of Frankenstein.

And interesting piece of trivia:
Boris Karloff became a father for the first time while filming "Son of Frankenstein." When he was told that the baby had been born, he immediately went to the hospital to see his wife and child while still in costume. When he arrived, in full Monster makeup and clothing, he created a panic throughout the hospital.