Basically, Percy had a... relationship with a mystery person while still at Hogwarts. Now, in this version of the wizarding world, there are spells and potions to allow men to bear children, but there's also the occasional wizard born with the ability to bear children naturally. Percy happens to be one of them, and winds up preggers during Chamber of Secrets, which he conceals from everyone. In a panic, he contacts a fairly recent Hogwarts graduate who he was friends with/helped him out when he first started, Nymphadora Tonks! And lo, there is a complex scheme in which Tonks's parents take care of Percy's kid while Percy finishes school (note: no, Percy does not inform the other parent, for various reasons).
Fast forward two and a half years later, and Percy gets knocked up again (by the same person). Now, though, he's out of school, so while he still conceals the pregnancy, he holds on to the kid (he also took back his firstborn). The war with Voldie is heating up, so Percy decides to flee to the States with his two kids. He winds up at a magic school, where he gets a job student-teaching and changes his last name to that of his mother's maiden name.
Meanwhile, Death Eaters kill Zacharias Smith's dad and cousin, leaving his cousin's baby daughter all alone, so Zacharias takes her in, and since Voldie seems intent on erasing all of the Smith family tree, also flees to the US. He winds up at the same school as Percy, where he finishes out his schooling and then also goes into the teaching biz.
Then there's Theodore Nott, who wandered his way over to the US mostly out of apathy to the war. He also winds up at that school, where he hooks up with Zacharias. They eventually have a kid, and then Theodore eventually dies because he forgot that green means go. Rite.
Anyway, back in good ol' England, Harry and the Hogwarts Six (the Trio + Neville, Ginny, and Luna) defeat Voldemort and win the war, etc etc. The Trio become professors, and the other three become Aurors. Ron and Hermione marry, but have no kids yet, Remus and Tonks married and had a kid during the war, ditto Bill and Fleur. That's about it.
Ellory Prewett, our main character! Daughter of Percy Prewett, aka Percy Weasley, and UNKNOWN. Slytherin first year, rather random but quite intelligent, although she can't apply it to anything.
Martin Prewett, Ellory's big brother. Ravenclaw fourth year, very intelligent and a bit mischevious.
Percy Prewett, previously Percy Weasley. New History of Magic professor at Hogwarts.
Zacharias Smith, the new Potions professor and Head of Hufflepuff.
Anna Smith, Zacharias's cousin's kid/his adopted daughter. First year Hufflepuff, highly suspicious and a bit of a revolutionary.
TJ Smith, aka Theodore Smith Jr. Theodore Nott and Zacharias Smith's child. Seven years old, incredibly hyperactive and obsessed with X-Men. Also tends to do Ellory's bidding because it amuses him.
Sirius Lupin, son of Remus Lupin and Nymphadora Tonks, also a first year Hufflepuff.
Jamie Mudge, Anna's fellow first year Hufflepuff. Good looking, charming, etc.
Caleb Lloyd, Jamie's BFF and Ellory's fellow first year Slytherin, who is the male representative to the Council. Becomes buddies with Ellory, but not all the time, because he does not humor her insane moments as Anna or Pluto would.
The aforementioned Pluto Thruston, another Slytherin firstie, and completely insane. But fun!
Christian Keller, aka Creepy McCreepsalot, another Slytherin first year. Mostly? He is creepy.
Valentine Dunn, the final male Slytherin firstie. Think Lockhart, only perhaps a bit more vicious.
Agatha Wilbin, a blood-purist Slytherin first year.
Hildegard Cuffe, Slytherin firstie whose father is the chief editor of the Daily Prophet.
Lothar Gelt, mysterious Welsh-speaking Slytherin firstie who should be in her second year, but isn't.
Patricia Biera, the seventh year female representative to the Slytherin Council. Despairs of the current batch of firsties.
Lazarus Bexax, male seventh year representative to the Council, likes to rely on his scariness, and doesn't know what to do with the fact that neither Ellory nor Pluto reacts to it. In fact, Ellory gets a crush on him. He is terrified.
Marcus Flint, Head of Slytherin and Care of Magical Creatures professor. Also the one who cast the deciding vote to make Ellory a Council member.
Harry Potter, Defense Against the Dark Arts professor.
Lisette Weasley, oldest daughter of Bill and Fleur Weasley, a first year Gryffindor. She likes to gossip, and finds curses to be "quite fun".
Grady, a first year Gryffindor who attempts to become Ellory's arch-nemesis, but it doesn't really work out.