Žalilac would be academic word although it is never used in above given context. This word will be used in official maner to signify an appeallant in official proceedings. We would rather use colloquial phrase "gunđalo" which has exact meaning of a person who is constantly complaining(a moaner)and is never satisfied. The phrase "Večito nezadovoljan(nezadovoljna)" means exactly a person who is never satisfied.