Salazaar Slytherin is the founder of Slytherin
You know that bloke who built the Chamber of Secrets, popped in a baby basilisk, stir, leave to simmer and wait a few hundred years and... wha la: Harry meets Tom Riddle!! Remember that Tom Riddle/ Voldemort is the last relative of Salazaar Slytherin therefore, Riddle/ Voldemort is the Heir to Slytherin.