
Grumbler
Negative Connotation
The word 'grumbler' often has a negative feel, suggesting annoyance with the person's behavior. 
Emotional State
'Grumbler' implies a persistent state of dissatisfaction that is emotional, rather than based on logical reasoning. 
Not Always Serious
Sometimes 'grumbler' is used lightly or humorously to describe someone who's known for their harmless complaining. 