The wildlings seemed to think Ygritte a great beauty because of her hair; red hair was rare among the free folk, and those who had it were said to be kissed by fire, which was supposed to be lucky.
George R. R. Martin (2012). “George R. R. Martin's A Game of Thrones 5-Book Boxed Set (Song of Ice and Fire Series): A Game of Thrones, A Clash of Kings, A Storm of Swords, A Feast for Crows, and and A Dance with Dragons”, p.1633, Bantam