I need an antonym (a noun) for the word "Critic", ie. a noun for someone who praises.
First I would like to point out that a critic says both positive and negative things. So the opposite viewpoint of a critic could be another critic.
However if you take critic to mean someone who is negative towards a topic, then I would consider advocate as a good antonym.
a person who publicly supports or recommends a particular cause or policy.
Flatterer or sycophant come to mind.
sycophant, n: servile, cringing, or abject flatterer; lickspittle.
But I should point out that critic can be a neutral term. Critics often praise. Movie critics do like movies from time to time and often say so.
panegyrist: a person who panegyrizes; eulogist.
More common antonyms might be fan, supporter, or devotee.
OS X dictionary
supporter |səˈpôrtər| noun
1 a person who approves of and encourages someone or something (typically a public figure, a movement or party, or a policy): Reagan supporters | supporters of the boycott. • a person who is actively interested in and wishes success for a particular sports team.