a person who is very interested in something

ardent supporter; fan

a follower of any leader of religious thought, art, learning etc (especially one of the twelve personal followers of Jesus Christ)

believer, follower

a worker who, in order to win favour, obeys orders too readily

sycophant

an animal or plant living on or in another and getting its food from it; a person supported by another and giving nothing in return

something that exists by taking from or depending on another

a person who accepts an offer from a person selling something

something that exists by taking from or depending on another