Key: Last Name, First Name - Book first appears in. Short summary.
Scabbers - Book 1. Ron Weasley's rat, who lived in the Weasley household for 12 years. Actually Peter Pettigrew in disguise.
Shunpike, Stan - Book 3. Conductor of the Knight Bus.
Skeeter, Rita - Book 4. I quote Rita's own quill: "Attractive blonde Rita Skeeter, forty-three, whose savage quill has punctured many inflated reputations" including Dumbledore, Hagrid, and Hermione Granger.
Slytherin, Salazar - Book 2. One of the Founders of Hogwarts. Left the school after building the Chamber of Secrets to clense the school of Muggle-born wizards.
Snape, Prof. Severus - Book 1. Head of Slytherin House and the Potions Master. Originally a Death Eater, Snape now teaches at Hogwarts, every year vying for the Defense Against the Dark Arts job.
Snuffles - Book 4. Yet another nickname for Sirius Black.
Sorting Hat - Book 1. An old hat that once belonged to Godric Gryffindor; it sorts all the students into their respective Houses.
Spinnet, Alicia - Book 1. Quidditch Chaser for Gryffindor.
Sprout, Professor - Book 1. Head of Hufflepuff House and Herbology teacher.
Stan - Book 3. See Shunpike, Stan.