Which ethnic group left the fair-haired blue-eyed characteristics now seen in the Spaniards in the northern regions?

The fair-haired, blue-eyed characteristics seen in some Spaniards, particularly in northern regions like Galicia and the Basque Country, are often attributed to genetic influences from the Celts. This ancient Celtic population settled in parts of the Iberian Peninsula and contributed to the genetic diversity of the region. Additionally, various migrations and invasions over centuries, including those by the Visigoths and other groups, have also played a role in shaping the physical traits observed in modern Spaniards.