Whinger
/ˈwɪndʒər/
Definition 1 of 1
Pro Tip 1/3Colloquial usage
Used in casual conversation among friends, not in formal settings. 
After the game, James called himself a whinger for complaining about the referee.
Pro Tip 2/3Negative connotation
Describes someone negatively, suggesting they're annoying or difficult to please. 
Our new colleague is a bit of a whinger; nothing is ever right for her.
Pro Tip 3/3British & Australian
Primarily used in the UK and Australia, may be less understood by American English speakers. 
When I lived in London, my mates called the complainer in our group the 'office whinger'.