Grumbler
/ˈɡrʌmblər/
Definition 1 of 1
Pro Tip 1/3Negative Connotation
The word 'grumbler' often has a negative feel, suggesting annoyance with the person's behavior. 
After the meeting, everyone complained about the grumbler disrupting the discussion with petty issues.
Pro Tip 2/3Emotional State
'Grumbler' implies a persistent state of dissatisfaction that is emotional, rather than based on logical reasoning. 
No matter how hard we tried to please her, the grumbler found something to criticize.
Pro Tip 3/3Not Always Serious
Sometimes 'grumbler' is used lightly or humorously to describe someone who's known for their harmless complaining. 
Uncle Joe is the family grumbler, but we all love his quirky rants about technology.