"I always knew Salazar Slytherin was a twisted old loony, but I never knew he started all this pure-blood stuff. I wouldn't be in his house if you paid me."
Ron Weasley[src]

Professor Salazar Slytherin was one of the four founders of Hogwarts School of Witchcraft and Wizardry. He spoke parseltongue and believed strongly in blood-purity.

He created Slytherin House as a bastion for those who upheld his ideals and for those of pure-blood, though it later years, not all who would be accepted into the house were of the purest blood.