#original 40

Theodore Nott - First mentioned by surname at the Sorting Ceremony in book 1, but not mentioned again or confirmed to be a Slytherin until book 5. For more about him and why he's on this list, check this post, which describes him better than I could ever hope to.

Adrian Pucey - A Chaser on the Slytherin Quidditch team in books 1, 2, and 5, who's never seen cheating or making any fouls, nor is he given any negative descriptors. He's also one of two Slytherin Quidditch players who isn't shown to be on the Inquisitorial Squad in book 5. The other is Miles Bletchley, who is sadly disqualified from this list for allegedly jinxing Alicia Spinnet.

Terence Higgs - The Slytherin Seeker in book 1, who is also shown to play fairly, and whose only negative description is that Harry is faster than him, which is to be expected seeing as Harry presumably has the better broom.

Peregrine Derrick - One of Slytherin's Beaters, he doesn't attempt to physically attack other players with his bat, unlike his fellow Beater Lucian Bole. Of course, he hits Bludgers at them, but that's just part of the game. He and Bole are described as unintelligent in book 5, but by Angelina Johnson, who as Gryffindor Quidditch captain would naturally be biased, and at any rate they've already left school at that point. On a side note, I feel kind of sorry for him and Bole, seeing as what would have been their final Quidditch season at Hogwarts was cancelled for the Triwizard Tournament.

Malcolm Baddock - One of the few Slytherins who are explicitly younger than Harry, he appears at the Sorting Ceremony in book 4, and is hissed by Fred and George Weasley, who are nearly adults by wizard standards at this point, for being sorted into Slytherin.

Graham Pritchard - Mentioned at the Sorting Ceremony in book 4, but doesn't directly appear. Presumably, he was just listed to fill out the scene, but he was sorted into Slytherin and is never seen doing anything evil or bloodist, so…

Daphne Greengrass - Mentioned as taking her OWLs in book 5. She's not explicitly stated to be a Slytherin in the books, but she is listed as one in the Original 40, which I consider canon to a degree.

Emmanuel* Vaisey - A Slytherin Chaser in book 6, he's stated to be their best goal-scorer. However, Gryffindor never plays against him due to him being injured by a Bludger during practice.

Magnus* Urquhart - Slytherin's Quidditch captain in book 6. He is described as crushing Harry's hand during a handshake, but I personally just see him as having a really firm grip, which isn't a bad thing.

Tracy Davis - Never appears in the books, but is listed in the Original 40 as a half-blood Slytherin, so I count her on this list.

Astoria Greengrass - Daphne's younger sister and Draco Malfoy's future wife, she never appears in the books and is never directly stated to be a Slytherin, but considering that her sister is most likely in that House and her future husband definitely is, it's very likely that she is, too.