[1] Team Torchwood (Jack, Ianto, Owen, Tosh, Gwen, continuity compatible until after 'Adam') find an unusual artifact, which de-ages them to 11 years old. Then they receive a Note to Self from Jack, which says that they are about to experience something wonderful. The artifact, a crystal, takes them to Jack's apartment during the Summer of 1993, whereupon they receive Hogwarts letters. Let the magic begin...
Anti-Hero: Almost everyone, of one variety or another.
Jack: Type III. Beginning to show signs of Type IV.
Owen & Ianto: Type III
Gwen & Tosh: Type II
Harry: Moving towards Type III.
Arson, Murder, and Jaywalking: Jack's Torchwood contract includes the phrases "high-risk situation", "dangerous artifact", "hostile forces" and "terrible flirt."
Also Umbridge. The narration of her arrest includes this gem:
She didn't even try to resist. In fact...she was crying. It might almost seem sad, if it weren't for the fact that she so deservedly brought this fate upon herself.
Bad Ass: Most of the main cast. Particular marks to Jack, who easily holds off Bellatrix with only Super Reflexes and 3rd year spells for 5 minutes, allowing Sirius to snipe her with a stunner. Moody notes admiringly that he has serious combat experience and that it took 12 top Aurors to take her down last time.
Moody references this incident later, saying its why Jack's one of the Seven Potters.
Badass Abnormal: Jack, being immortal is this by even Wizard's standards.
Berserk Button: For Jack, the words 'Master' and 'Freak'.
"We don't use that word in the Dungeons anymore. Jack maims us for it." This from Deatheater Draco Malfoy.
Beware the Nice Ones: Ianto. Just because he happens to be in Hufflepuff and is apparently harmless doesn't mean he isn't extremely dangerous, especially when he shows just how easily he could poison the entire school.
Everyone smart is absolutely terrified of Ianto.
Jack. People underestimate him at their peril.
Bi the Way: Jack, since no alien species have turned up to provide credence to his omnisexuality.
Theo Nott.
Draco Malfoy.
In a bonus scene, Jack describes Draco as straight, even while seducing him.
Harry. Sort of. He's perfectly happy to kiss Jack just to see what kissing is like, but Jack dissuades him because Harry is Like a Son to Me.
Blatant Lies: Jack claims that he's a vampire to escape a photographer. Team Torchwood doesn't deny that Past!Jack being Present!Jack's father is this.
Brain Bleach: After talking to John Hart, Snape really needs a good Obliviator.
Dumb Muscle: Many Death Eaters, an opinion shared by both Jacks and Lucius Malfoy.
Doing in the Wizard : John Hart's explanation of magic as understood by the 51st century. Yes, John Hart is a wizard, and a powerful one. Tremble ye mortals!
The Don: by Deathly Hallows, Owen is the undisputed Don Corleone styled ruler of Hogwarts.
The Dreaded: Voldemort and as of Chapter 73 Owen, known by the entire school as 'Owen Bloody Harper' after being the chief architect of Umbridge's downfall and mental breakdown.
Enemy Mine: A less serious one between Ron and Malfoy when dealing with Umbridge.
A deadly serious one in Chapter 86 between Harry and a not quite dead Draco Malfoy.
Enfant Terrible: The general public's perception of Owen Bloody Harper.
Everyone Has Standards: Someone even Jack won't do? Umbridge. Owen and Tosh revel in the incredibility of this.
Future Me Scares Me: More the possibility as shown by the Boggart. Of course, Jack being Jack, he flirts with it.
"Funny Aneurysm" Moment: In-Universe. Jack jokes lightly about Dumbledore's attempts at getting in his head, after the third task, comparing it to sexual assault. Then Harry sort of curls up and says Voldemort essentially mindraped him. Leads to a Crowning Moment of Heartwarming.
Half-Human Hybrid: Hagrid, Madame Maxime. Jack jokingly suggests that Snape is half Dementor and Owen claims Derrin Harper is part name stealer part mountain troll.
Manipulative Bastard: Jack again. Witness how he plays the Wizarding World like a harp over Malfoy's death (and making it happen) until one of the DA proves to be the spanner in the works, in which some Xanatos Speed Chess follows. Also note how he draws Draco and Harry to him and gets them to work together.
Theodore Nott.
Memetic Badass: Owen in-universe, considered to be one of the few people who could conceivably deal with Voldemort after dealing with Umbridge.
Mind Screw: The Fidelius charm on Jack's immortality has this effect.
Operation Jealousy: Ginny tries to pull one on Harry via Jack. Jack, being walking Genre Savvy, sees through this, and gives her some fatherly advice. Ginny and Harry hook up. Just asJack planned.
Secret Keeper: Jack and Ianto via the Fidelius charm
Self Cest: Jack. With his boggart. This surprised absolutely no one.
She Is All Grown Up: Mentioned occasionally (especially in reference to Torchwood, who have all been de-aged), such as Gwen, who is noted by Jack in a She Cleans Up Nicely moment that she looks more like 15 than 12 during the Yule Ball and passing for 16/17 at the Department of Mysteries at age 13/14.
Shout Out: Torchwood One are working on Project: Rebirth. And Jack has mentioned in the past that they tried to build a supersoldier out of him in the 70's.
Snark Knight: Team Torchwood, Draco and Harry (less than the other two).