The Persian is a mysterious figure from Erik's past. He knows more of the history of the Opera Ghost than any other character. The Persian leads Raoul into the Phantom's lair in the original novel, and warns him to "keep his hand at the level of his eyes". He also saved Erik's life at some point, and would seem to almost consider himself Erik's friend were it not for the constantly opposed moral views they have.
In Phantom by Susan Kay, he plays an even larger role. he is the Royal Daroga of Persia. The Daroga, as explained in Leroux's original book, is the chief of police. In Kay's book, he is also given a name, Nadir Khan. Erik and himself were friends in the Persian court. Erik entertained his son and helped him in the last few months of his life. When Erik is to be arrested and sentenced to execution for knowing too much, the Persian risks not only his position but his life and allows him to escape. He only survives because his servant placed a dead body on the shores of the sea and dressed it in Erik's clothes and mask. The body was unrecognizable, so the Persian was not killed. Instead, he was exiled. But, due to his distant royal lineage, he was allowed a pension and came to live in Paris. When he once again meets Erik, he begins taking notes on him.
It is a symbol of how well Erik thought of the Persian that he actually tried to keep a promise he made to him. As Nadir was releasing him years ago, he made Erik promise to commit no more pointless murders. Not only did Erik agree to the promise, but he tried, over the years, to keep it, and all but succeeded.