Minerva McGonagall

McGonagall is competence with a bun. She enforces rules, bends them for a seeker when the house needs one, and has very little patience for ministers who think a school is a prop. Students fear her and want her on their side, which is the correct reading of a formidable teacher.

She is an Animagus — a cat — a fact the opening chapter lets you see before it explains. The journal files that as story reference, not as a twist to protect.