I'm a Con Artist in HP

System: Go on, earn emotion points and deception points.

Zhang Moxun: Sure thing.

And so...

Professor McGonagall: A first-year Animagus? Impossible!

…

Zhang Moxun: Dumbledore! I want to learn the Three Unforgivable Curses.

Dumbledore: …All right, you can go to the Restricted Section. Whether you learn them well is up to you.

A week later, Dumbledore: Oh, Merlin! I shouldn’t have let him study them. How can a first-year student learn the Three Unforgivable Curses?!

…

Snape: Come to my office after class and serve detention.

…

The protagonist isn’t especially close to the main group. They greet each other and lend a hand now and then, but they don’t spend every day together.

He shares a room with Draco, so they’re always running into each other. They get along well enough, but Draco is the only one who thinks they’re close. In reality, they aren’t.

The protagonist doesn’t turn into a stupid child just because he becomes younger. His intelligence doesn’t decline. Everything he does is for personal gain.

The protagonist has no romantic tension or relationships of any kind beyond friendship.

(Crossover: Detective Conan, The Grave Robbers' Chronicles, Bungo Stray Dogs, and more may be added later.)