"Rats can be wizards? Or is it 'wizards can be rats'?"

"Some wizards can turn into animals, McGonagall does it. Are you saying Scabbers was an animagus this whole time? And who are you anyway?"

"Remus Lupin, I'm the new Defense professor. And yes, I'm afraid so."

(Really? They couldn't have a new Defense professor for an entire day without someone who's supposed to be dead turning up?)

"Why would someone stay a rat for--what, ten years?"

"Twelve. Twelve years." But Lupin isn't looking at Ron; he's back to staring at the models in anguished confusion.

"Have you had him this whole time? How did you get him?"

"Percy had him before me. He found him in the garden, and mum let him keep him, and then I got him when Percy got an owl. He always seemed like just a normal rat. I guess it was odd that he lived so long"

"Okay, uh, maybe whatever apparently didn't kill him did hit him really hard on the head?" Cam suggests.

"Maybe. I should go see if--if--" he stands up, jerkily, and starts walking toward the door, not quite steady on his feet.

"Do - you need company -?"

Answering that question would require understanding it and putting words in an order and speaking them, so it's not happening. Marionette-walking out of the library is what's happening.

"I'm... sorry about your rat. He didn't act weird? Just lived a long time?"

"Mostly he just slept a lot. I should, uh, go get him. Or try to get an explanation out of Lupin or something."

"Do you want company or nah?"

"Um, it'd be cool if you came? Scabbers might have gone somewhere else while we were talking. It's fine if you'd rather not though."

"I don't mind." He follows Ron.

So do Harry and Hermione. It's not hard to catch up with Lupin, and he doesn't seem to mind the company.

They go up a couple floors and down a couple hallways and there's that suit of armor, distinctive axe and all.

When Lupin levitates the armor aside, there's the rat behind it, wide-eyed and motionless.

"Peter? It's me. Are you alright?"

"What's the matter? Why didn't you tell anyone you were alive? You were safe--unless . . . unless they switched. Oh Merlin, Peter, just tell me it wasn't you!"

The rat dodges around Lupin's legs and bolts down the hallway, Lupin in hot pursuit a moment later.

"I really wish you'd told me what was going on so I would know whether to help catch him!" Cam says, exasperatedly, following at the briskest walk he can.

Lupin is still not super coherent. He yells "Traitor!" and "Murderer!" and then remembers he's a wizard and starts throwing bolts of red light. A couple of first-year Hufflepuffs emerge from an intersecting corridor, take in the scene, and head back the way they came from. 

Scabbers or Peter or whoever is a small and moving target, but there's not really anywhere to hide in this hallway; soon rat and human skid to a stop, the former unconscious and the latter panting.

"You wanna catch us up? So we know how disturbed to be about the decade as a childhood pet thing, if nothing else?" Cam wonders, when he gets within speaking distance.

Lupin stares at Cam for a bit, blinking like he's trying to remember where he is and how to form sentences. Eventually he starts talking, looking from Cam to the rat on the floor every few seconds as of to reassure himself the latter is still there.

"I can try. It's a long story, and I just found out it's even longer than I believed, but I can tell you what I thought I knew yesterday, at least."

"We were all friends, at Hogwarts, me and Lily and James Potter--well, she was Lily Evans then--and Peter, and--Sirius Black. And when the war came, we all fought together against You-Know-Who. And Lily and James fought as well as anyone, better, they were amazing, and it painted a target on their backs. And then they had Harry."

(Harry and crew have caught up with them, and are listening in variously flavored bewilderment.)