Teachers

Transfiguration teacher, head of Gryffindor house, and a registered animagus cat,

The keeper of the keys, Hagrid was expelled in his 3rd year, takes up teaching care of magical creatures in Harry's 3rd year.

Severus Snape, responsible for Ron's life, was a death eater- until Voldemort threatened Lilly Potter's life. He then switched over to the order, although not publicly. He still worked for the Dark Lord, but as a double agent for Dumbledore.

The charms teacher at Hogwarts, Flitwick is a tiny little wizard with some goblin ancestry. He is a very skilled wizard.

Pomona Sprout, the herbology teacher at Hogwarts, is a kind professor who is an expert in all magic plants.