The proper response to someone who says "Jazakallah Khair" is "Wa iyyaka" for male, "Wa iyyaki" for female, and "Wa iyyakum" for plural. This means "and to you too." Another more formal response is "Wa antum fa jazakumullahu khayran," which means "And you too, may Allah reward you with goodness". It is important to note that "Jazakallah" is an incomplete phrase, and it is better to say "Jazakallah Khairan" to convey the full meaning of "May Allah reward you with goodness".