Sirius Black is on the loose and the Ministry has decided it would be prudent to assign Harry Potter a bodyguard. They want to choose one he'll never even suspect, and who better than everybody's favorite metamorphmagus.

Sirius Black is on the loose, and the Ministry has decided it would be best if Harry potter were well protected. And so enter Nymphadora Tonks, the Ministry's youngest Auror and only Metamorphmagus. Or, perhaps you should call her Erin Davies, that's the name Harry will know her by as they go through their third year at Hogwarts together.