Born to Johanna and Leonardo de Castile, Valentín de Castile had a pretty easy life growing up. His father was a merchant, a trader of silks and dyes. His mother, a seamstress to a variety of noble Spaniards. The couple did well, until of course, the economy dropped. No one could really afford silks, so they switched to more modest materials, namely cotton, resulting in a lack of demand for what the Castiles were offering. This resulted in the family hitting hard times, and struggling to put bread on the table.
It was here, that Valentín discovered his alchemical abilities. He was determined to find a way to help get his parents' finances sorted again - so her turned to magic. Both his parents had magic, but neither were particularly keen practitioners, so they struggled in comparison to the natural talents of their son. He turned to his books and his magic, and found himself getting a knack for transforming base metals into gold. Valentín was able to use his abilities to pay off his parents' debt.
With his parents' debt paid off, all that was left for Valentín to do was to make his own way in the world. He didn't like the socio-political situation in Spain, so he decided to make a move towards the West: particularly, to Hogwarts School of Witchcraft and Wizardry. Believing that it was important for him to pass on his vast knowledge about alchemy, he decided to teach at the school.