They Shook Hands is a fanfic that tells the overplot of the Harry Potter series using the Slytherin characters.May contain spoilers!
Harry PotterStill the hero of the story. His encounter with Draco in Diagon Alley was where everything started to change for the better. Harry exchanges letters with his new friend in the month before school starts, and on the train, Draco introduces him to some other children.
Draco MalfoyStill an obnoxious snot. He just happens to be friends with Harry. The reader can almost imagine that this is how he would have behaved had Harry accepted his offer of friendship on the Hogwarts Express in canon.
Theodore "Theo" NottThe smart one of the group. His father is a scholar, and it's definitely rubbed off. Smug and extremely prejudiced about Muggleborns.
Pansy ParkinsonThe leader of the Slytherin girls in her year, she also exerts a powerful influence with other forms. Described as pretty enough, she's not drop dead gorgeous, but she's got the most important family (of the girls). She and Draco were betrothed an unspecified number of years ago, but neither of them seems to take it particularly seriously. She doesn't seem interested in him romantically until Harry tells her that Draco likes her but later in the story they do date.
Tracy DavisThe Betty to Pansy's Veronica — sorta. She's the first girl to reach out to Harry and make him feel comfortable. She gives him his first hugs and invites him to talk about anything he wants to. She also tells him that he doesn't have to agree with everything they all say in order to stay friends. She develops a wicked crush on Harry, which later causes problems since he wants to just be friends.
Millicent "Millie" BulstrodeMillie doesn't mind grubby robes and getting dirty. She rebelled against the overbearing nature of her mother the fashion designer.
Daphne GreengrassThe snarky one of the group. Daphne always has a zinger and loves to get in the last word.
Vincent CrabbeOften used for comic relief. A Running Gag is that he has a fat head, leading to his inevitable protestation to the contrary. Given further dimension from the canon by being good at Astronomy.
Gregory GoyleOften thought of as Crabbe's identical twin, in They Shook Hands, he's a much more distinct character. Thinks of himself as a "Man of Action".
Laine SlaterThe Slater twins are a year behind Harry. She's got a crush on Harry, but she doesn't let that get in the way of being friendly with him.
Ron WeasleyWithout Harry and Hermione as friends, Ron is a more extreme version of his canon self. He tries to make friends with Neville, but things don't work out so well. Most often seen in the company of Dean Thomas and Seamus Finnigan.
Neville LongbottomIn Y1, Harry got the Slytherins to stop picking on Neville, as he is a pureblood and to harass him goes against the Slytherin house rules. This means his only nemesis is Professor Snape and Potions class. His development is a bit different, and by the time we see him in Y4, he's a stronger character than in the canon.
Chrissy GoldingOne of the two missing Gryffindor girls from canon. She went with Ron Weasley to the Yule Ball. Used along with another OC, Amy Geagan, to flesh out the different elective classes shown.
Hermione GrangerBecause of her rudeness to Harry on the train, the proto-Slytherins conspire to bribe the Sorting Hat to send Hermione to Hufflepuff. This is a plot device for Dethryl to push the story in a certain direction, but it does make sense because Hermione works harder than anyone (she had her schoolbooks memorized before the train). Later in the story, her suitableness for Hufflepuff does come up, and through the Sorting Hat itself, the author makes the case.
Padma PatilIn a canon inversion, Harry asks Padma Patil to the Yule Ball. She accepts, and they have a great time.
Severus SnapeHead of Slytherin House. He tries to be a mentor to Harry. Though prepared to hate Harry for looking like James, when Harry is Sorted to Slytherin, he is forced to think again.
Albus DumbledoreHarry and Dumbledore don't get along. Harry has a towering resentment for being dumped at the Dursleys, and when it comes out that Dumbledore facilitated that, there's a huge row.
VectorThe Arithmancy witch is a former Slytherin, and she has helped Harry on several occasions.
Sirius "Padfoot" BlackHarry's godfather. Due to the way the plot of Y3 was resolved, he is absolved of his crimes and able to offer Harry the proper home he's always wanted. He is teaching Harry to be an Animagus.
Remus "Moony" LupinHarry's former professor and friend. In addition to teaching Harry the Patronus Charm, he has also shown him the Stunning Charm.
LuciusThis characterization of Lucius is definitely not typical. He's a bad man, but he may not be an evil man. He has a good side to him, but his moral code is different. Very much a Godfather sort of criminal. He's Harry's strongest positive male influence for the first three stories, and even with Sirius back in the picture, Lucius has a significant role in things.
NarcissaA bit of an emotional woman, she loves her family dearly and will ruin anyone who threatens it. She's portrayed very similarly to the canon but given more depth.
ElanThe functional point of departure from the canon takes place ~1976 with the birth of Elan Octavio Malfoy. He's the leader of his peer group, and he's better with people than Draco was in the canon.
Tonks familyNot a big part of the story yet, but Harry did go for dinner at the Tonks house.
TedNot much is known about Ted in the canon, so the author is going to have to make up a lot given how he's brought the Tonkses into the storyline.
AndromedaSirius' favorite cousin. Once he is exonerated, he reconnects with her.
NymphadoraSo far, she's her canon self. Hopefully we'll see a lot of her in Y5.