Devils are famous for enjoying making contracts, often with normal denizens of Materia. These are known to grant great boons or power to the contract-taker, but at some cost or in return for some work or action. Many claim these contracts are magically binding, though it is unknown if there is some greater magical force that forces a Devil to be true to the law. Another perspective is that it is simply an elegant tool to convince or trick people into submission, in reality backed up with pure (and often magical) force.
Many have wildly speculated that the Philosophers of the Round and Knights of the Round often have such wild physicality, such extraordinary minds and such impossible luck that surely many of them must’ve taken Devil Deals to get there.
One Devil which has made many deals with people on the verge of death is the Good Doctor.