Complainer (noun)

1. A person who complains frequently or habitually.
2. A person who expresses dissatisfaction, pain, or resentment usually vocally.

Origin:

From the verb complain + -er.

Examples:

1. He was known as a constant complainer, always finding something to complain about.
2. She was tired of being surrounded by complainers who never seemed to be happy.
3. He was labeled as a complainer by his colleagues for his constant negativity.
4. The complainer in the back of the class was disrupting the lesson with his complaints.
5. The complainer at the customer service desk was demanding to speak with a manager.