moody person; temperamental person; fickle person
Describes someone whose mood or attitude changes easily, like the weather. Often used in a light, colloquial way.
See also: 天気屋
He's a moody person, so he sometimes suddenly gets into a bad mood.
That person is fickle; what they say today is different from yesterday.
Also means a moody person, but 気分屋 emphasizes being driven by one's feelings of the moment, while お天気屋 likens the changeability to weather.
Compound of お天気 (weather) and 屋 (person with a certain trait), literally 'weather person', referring to someone whose mood changes as often as the weather.