After reading Holly Tucker's City of Light, City of Poison, I found myself riding a wikipedia blackhole that started with the Sun King himself, and ended with the Man in the Iron Mask. What are the leading theories on the prisoner's identity, and is any serious credence lent to the idea that he is The Sun King's twin brother / true father / ex-lover?
There's no definitive answer to this question, but I discussed the background to the case and the available evidence in an earlier answer that might be of some interest to you:
The idea that the Mask was a relation of Louis XIV dates to an anti-French Dutch pamphlet published in the 1690s, and was also picked up by Voltaire, but there's no evidence to back it. It's based on the idea that the prisoner must have been someone extremely eminent, to be so dangerous as to warrant long term imprisonment, and extremely recognisable, to require being masked. However, the evidence we have suggests that the mask was worn only when the prisoner was being transferred between prisons, not on a day to day basis while incarcerated.