FAN
fan 2 [fan] (plural fans) noun

1.  enthusiastic admirer:  an enthusiastic admirer of a celebrity or public performer

[Late 19th century. Shortening of fanatic. The word is first recorded from the 17th century, but it was not used regularly until the 19th century (referring to sports supporters).]

See fanatic n.2:

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FANATIC
fa·nat·ic [fuh náttik ] noun (plural fa·nat·ics)

2.  fan of something: somebody who is very enthusiastic about a pastime or hobby. Also called fan

[Mid-16th century. Directly or via French from Latin fanaticus "inspired by a god, frenzied," from fanum "temple" (source of English  profane).]

Ant. See detractor

DETRACTOR
de·trac·tor [di tráktr ] (plural de·trac·tors) noun
adverse critic: somebody who disparages or devalues something or somebody

See:  critic (3)

CRITIC
crit·ic [kríttik ] (plural crit·ics) noun

3.  somebody who finds fault:  somebody who does not like something or somebody, or who finds fault with something or somebody

[Mid-16th century. Via Latin from Greek kritikos "discerning," from krits "judge," from krinein (see crisis ).]