All the pieces of the puzzle finally fit together, and I was almost 100% sure that these were the real facts.
"I would like to know one thing: why did you go after the Fat Lady on Halloween?"
"I had spent 12 years in prison while he was still alive, and after tracking him down and traveling miles to reach Hogwarts, I couldn't enter the common room because I didn't have the password. I was frustrated because I was so close to him yet still so far away. I was afraid he would run away or attack my godson. I had too many negative emotions that made me react impulsively."
The feelings he had were understandable. I think a lot of people would have reacted the same way.
He was at the castle that night because he had the common room passwords.
He met Crookshanks some days after Halloween, and even though the cat was initially skeptical, it eventually trusted him.
Hermione's cat is actually very intelligent; he knew from the beginning that Ron's rat wasn't really a rat, which is why he had been chasing him since the start of the year.
Crookshanks had stolen a piece of paper containing all the common room passwords — most likely Neville's — and given it to Sirius.
Sirius decided to take action that night before the passwords changed.
The caretaker of the common room initially refused to let Sirius in, but after Sirius recited all the passwords for the week, the caretaker allowed him to pass.
The outcome was predictable since Ron's rat had been missing since the start of the week, so when Sirius showed up hoping to finally kill Peter, he was nowhere to be found, forcing Sirius to give up.
The problem was that Ron had woken up at that moment, and now the castle was probably being searched from top to bottom.
The second problem is that Peter might no longer be in the castle and could already be in another country.
"You can count on my help to find Peter, but the problem is that even if I ask Marie, we will only be four searching for him. We need to unite."
"And how do we make sure he doesn't alert the Dementors?"
"You were friends with Professor Lupin, so you could try to tell him the whole truth and about Peter's betrayal."
"He'll never listen to me."
It's true that Professor Lupin might not believe him, so we needed a way for him to hear Sirius's entire story without them actually meeting.
"I think Professor Lupin has some doubts; otherwise, he would have warned all the teachers that you're an Animagus, and this would be known throughout the school."
The simple act of not revealing it could tip the scales in our favor.
"What we're going to do is simple: you'll write a letter to Lupin with your signature, and I'll deliver it."
He finally agreed, and even though his handwriting was very poor due to the lack of practice in prison, he still included some facts that only he would know to prove that he was the one who had written the letter.
"I have a question, Sirius. By any chance, were the Marauders you four?"
"Yes, even if I can no longer consider Peter one of us, we were the Marauders. How did you know about our existence?"
"I know two people who have a map of the castle that you made, and I'd like to know if it's possible for an Animagus to appear on the map even when they're transformed."
"Of course! The map is never wrong. Even if the person had taken Polyjuice Potion or was under an invisibility cloak, they would still appear on the map as long as they were in a location covered by it. So, this room I don't know about wouldn't be part of it."
Once he had answered all my questions, I left him there so he could wait for things to calm down a little before leaving tomorrow.