Unlike many other forms of Agents who draw the majority of their powers from within themselves or a form of magical artifact, Contractors attain abilities through the formation of "Contracts" with higher powers. Each contract is a two sided agreement in which the Contractor gains control of a particular power however must also sacrifice something in return. Typically payments come in one of three forms: An immediate payment, which can be anything from a treasured item to a body part, an Obeisance, which is a small payment after each time an ability is used which can be as simple as drinking a glass of water to reading en entire book, or a later payment, which usually comes in the form of some herculean task or trial to be completed later in life. Often times the size and scale of payments is relative to the powers given however this isn't absolutely always the case. In theory, Contractors can take from one to an infinite number of contracts though most stop between one and five as the costs begin to outweigh the gains. The powers gained through a contract are extremely varied though most seem to center around the elements and/or physical augmentations.
*Many and varied. (Basically whatever you want as long as it's not OP and makes sense)
Powers are limited to one per contract. While this may seem constricting, multiple contracts may be taken and each power, given adequate thought, can be applied to produce interesting outcomes. Especially when combined with other powers.
Payments are proportional to the power given, however how much a payment is worth can often be subjective in a sense. For example, an individual who is barely willing to make said payment may get a lot more out of it than one who would do so without a second thought.
Contractors are human, so they are just as susceptible to forms of physical/mental harm as any other. Some contracts invalidate this and provide additional forms of protection or augmentation.