Louis Cypher

Louis Cypher  was a major character in the 1987 film Angel Heart in which he was played by Robert De Niro. Despite his human appearance, Cypher is in fact the Satan himself ("Louis Cypher" is a pun on "Lucifer").

Biography
At some point, an aspiring singer by the name of Johnny Favourite became intrigued with a group of Devil worshipers. Somehow (possibly through black magic) Favor was able to meet with Satan and make a deal; he sold his soul for fame and fortune.

For a time, Favourite got his wish, and became a huge singer. However, he eventually began to develop second thoughts on his deal. Seeking to cheat Satan, Favourite discovered a ritual which he believed would allow him to steal another man's soul. Favourite chose a soldier named Harry Angel, brought him to his apartment, and proceeded to kill him and then eat his heart. It is not known whether this actually managed to temporarily hide him or if Satan knew all along and merely played along hoping to lure him into a false sense of security.

Favourite was drafted into World War II, but was ultimately brought back to the United States after suffering severe facial injuries. In the process, his mind began to take on the memories of Angel, and after some facial reconstruction changed his appearance, he became convinced of his own identity.

Twelve years later, Satan, possibly in an act of mockery, took on the pseudonym of "Louis Cypher" and proceeded to approach Harry Angel, and gave him the case to find Favourite, telling him the truth, but wording it in a manner that could be misunderstood, namely giving the impression that he was simply a business man and that the missing person owed him money.

Every step of the way Satan worked to manipulate Angel, who he knew from the start was actually Favourite. He secretly influenced him into killing those connected with Favourite's past, causing him to run into trouble with the police when he became the prime suspect for their murders. Angel eventually discovered the truth on his own, but by the time he did it was too late for him to back out, and Satan finally revealed himself. Angel was later arrested and presumably executed for the murders he committed, fullfilling his debt to Satan.

Trivia

 * Robert De Niro's performance is actually an impression of popular director Martin Scorsese.
 * Marlon Brando was considered for the role of Louis Cypher before De Niro was cast.