chronic complainer; person who makes unreasonable complaints
Wasei-eigo from English 'claimer'. Refers to someone who habitually complains, often unreasonably, especially to businesses or service providers. Carries a negative connotation.
wasei-eigo (Japanese-coined English), from claimer
That customer is a chronic complainer, so be careful when dealing with them.
We get calls from unreasonable complainers almost every day.