Every Saturday of 5th Year, Sirius would take on his animagus form and go out into the Forbidden Forest.
The third Saturday he did this, Sirius saw a white fox. As he approached it, planning on scaring it away, he realized it was another animagus. He began to speak to her.
The fox told Sirius all about her and, being as clever as he was, figured out this cat animagus was Marlene McKinnon.
He didn't tell her he knew that. Neither asked for the others' name. They just called each other Snow and Night.
It stayed like this for several months, up until the Sunday before break. Sirius climbed through the portrait hole that evening, not having seen Marlene since last night. Marlene was studying in the common room when Sirius passed by.
"Hey, Snow" he smiled.
"Hey" Marlene replied. Sirius continued walking, not realizing the secret he just gave away. Not until Marlene cornered him two minutes later, having figured it out. "Sirius Orion Black III! It was you?"
"Whatever are you talking about, 'Kinnon?"
"Don't play dumb, Night. How long have you known it was me?"
Sirius smirked. "Since the first night."
"And you never told me!?"
"Correct."
"Why?"
"I wanted you to figure it out yourself, McKitten."
"If you wanted me to figure it out myself, you shouldn't have called me 'Snow'!"
Sirius, who'd been taking a sip of water, started choking, not realizing he called her that. "Shit! They're gonna kill me!"
"Who?"
"Uh, one second." Sirius bolted up the boys' staircase.
Inside, we're already James, Remus, and Peter. "Guys... I made an uh oh."
"What on earth did you do, Padfoot."
"Marlene knows I'm an animagus."
Shouts came from James and Peter while Remus shushed them. "Sirius, explain" Remus commanded.
And so, Sirius explained everything that happened over the past few months. At the end of his story, all three boys were laughing.
"What? What's so funny?" Sirius questioned.
The other three boys said at the same time, "Paddy's in lo-ove!"
