Recaller: A person who recalls ie. summons a nymph and binds them in a contract. contracts    vary with different veneers and elements, so there's no telling what the nymph will ask for as a price. it's usually really hard to tell, anyways. 

Nymphs: Nymphs are creatures of the elements. There are four elements: Wind, Water, Earth and Fire. Different elements have different personalities and demeanors. Wind types normally do things based on their mood, they don't give a what you think. Water types are calm, very intelligent and can most often outbargain anyone into a contract which suits them better than the recaller themselves. Earth types are more feral, harder to contain, but are also very loyal to their recaller. A recaller can recall up to five or six nymphs. A recalled nymph can also be recalled by another recaller. Fire types are very rare and their behaviour streaks are currently unknown. 

Levels: A nymph's levels are from one to six, with six being the highest rating and one the  lowest. 

Veneers: The main aspect of nymphs and recallers. For instance, Dongwook has a masking veneer. He can mask his presence from others and stay hidden, which is more defensive than offensive. Recallers can only recall nymphs with the same veneers.

Contracts: Contracts are formed when a nymph is bound to the recaller by bargaining their prices and tasks. Prices and tasks vary and will be bargained according to the nymph's ability. A bound nymph is usually incorporeal, ie. they mess around from the peanut gallery that is the recaller's brain/mind. Incorporeal contracts are easier to control, and nymphs only show up when called. What Seunghyun is attempting is a corporeal recalling. Corporeals are often very powerful and rank high on the level scale. If the nymph is bound, then it will have to stay with Seunghyun for sure. 

Tablo: A water elemental. Bound to Choi Dongwook. Will give the impression of fish scales on skin when recalled. Bargain price unknown.

Tukutz: An earth elemental. Bound to Choi Dongwook. Gives the impression of bark on skin and leaves in hair when recalled. Bargain price is blood with plants. 

Seungri: A wind elemental. Bound to Choi Seunghyun.  Gives the impression of black feathers and talons when recalled. Bargain price unknown. 

Bom: An earth elemental. Bound to Choi Seunghyun. Gives the impression of hair swaying gently in the spring breeze and flowers in hair. Bargain price is paintings of pretty things.