This one-time Potions Professor, being of a portly persuasion, had his own personal access provided to the Dungeons level, seeing as how he didn't want to use the Potions Professor's time-honoured Office. This staircase was first mentioned in "the Half-Blood Prince", when Luna Lovegood went down these stairs to attend the Slug Club Party.
Magnus and co. were back early from the holidays, and he had been using this time to revise for his NEWTS explore the castle. This staircase was particularly quiet, so he took his small copy, worn of the communist manifesto (that he had bought in a charity shop for 99p, no less) out of his pocket and begins reading section two, 'Proletarians and Communists'.
Kell's finished unpacking by now, and he's gotten bored with the schoolwork he'd procrastinated over Christmas, so is on his way down to the Quidditch Pitch for some practice. "What've the bourgeoisie done now?" he asks, spotting his brother and the familiar book. He's not at all political, at least compared to Magnus, but he's decided he's going to support all his siblings in any of their pursuits and, hey, if that means becoming a communist? He's down.
"Oh my god, Kell. They literally obliterated our entire concept of wealth distribution and there's no way we'll ever recover." He says, smiling at the sight of his baby brother. "Whatcha doing?" He asks, as he folds the top corner of the page and sticks the book into one of his robe pockets.
He chuckes, "You're right. But...Before you go." He says, digging into his pockets and getting out a neat little box . "Merry Christmas, Princess Kelle." he says with a smirk.
"Technically I'm your only brother, but I'll take what I can get," he laughs after opening the box. "I love it, thanks. Yours is in the common room still, though, so you're gonna have to wait."
