To the anonymous user above: even if it IS pronounced REH-yes, which it is, it still LOOKS like Reye's syndrome. All I think of when I see this name is how I read something about Reye's syndrome when I was little and it freaked me out.
This is a Spanish name that has nothing to do with the Magi (Three Wise men) of the Bible. Our Lady of the Kings (La Virgen de los Reyes, Santa Maria de los Reyes) is a marian advocation of the Catholic tradition. She's actually the Patron Saint of the Andalusian city of Seville and her 12th century image is venerated at the Cathedral main altar. According to the legend, the Virgin Mary appeared to King Ferdinand III in 1248 on the eve of his battle against the Muslim king of Seville announcing him his victory over the enemy army. Later, the pious king summoned artists all over kingdom of Castile to fashion a sculpture of the Virgin exactly as he has seen her in her miraculous apparition, so that he could have it for his private cult. Two angels, disguised as foreign artisans, came to his court and accomplished the king's wish. [noted -ed]
In Spain, this name is most common in the province of Sevilla. About the masculine/feminine thing: According to the official statistics, there are 6070 women named Reyes in Spain, and there are 1530 men named Reyes, so it is predominately a feminine name, but like many names which honour the Virgin Mary, it is occasionally given to boys.