Salazar Slytherin
One of the Four Founders of Hogwarts School For Witchcraft and Wizardry
Merlin
Former Student at Hogwarts, and a famous wizard
Phineas Nigellus Black
Great-Great Grandfather to Sirius Black and most hated former Headmaster
Tom Marvolo Riddle
Former student at Hogwarts and Lord Voldemort himself
Severus Snape
Head of Slytherin House, former Potions and Defence Against the Dark Arts Proffessor, and former Headmaster
Horace Slughorn
Retired Potions Proffessor, but is brought back to teach in Book 6-7, and Former Head of Slytherin
The Bloody Baron
Slytherin House Ghost, and the only one who can control Peeves