Satan

First appearance: Prince of Darkness (1987)

In the bowels of a small run down church a secret is hidden. A group of physics students spend the weekend trying to discover what that secret is. Turns out its a big can of liquid Satan and Jesus was an alien. Can the students stop the awakening evil from bringing Anti-god across the dimensional plane and destroying the universe?

The second movie in John Carpenter's Apocalypse Trilogy (the other two are The Thing and In the Mouth of Madness)

Herbert West

First appearance: Re-Animator (1985)

A scientist attempts to discover the secret's of life but runs afoul of a jealous professor. With the help of his roommate he continues his work in secret. Mayhem ensues.
One of my favorite horror flicks. It originally inspired a desire in me to become a doctor. I met Jeffery Combs at a comic convention once and told him this. His response?
"You're a sick, sick man."
Dr. West is still one of my favorite characters.

Jan Compton

First appearance: the Brain that Wouldn't Die (1962)

I'm sure you've heard the story a million times before. A surgeon and his wife having a car accident.  Surgeon takes wife's severed head and reanimates it in a pan in his private laboratory. Surgeon goes to strip clubs looking for a fancy new body for decapitated wife. Wife becomes psychically linked to weird conehead with a saggy butt created by surgeon husband. Wife becomes seriously depressed.