Derry stared at Selwyn uncomprehendingly. He didn't understand half the words that came out of his mouth, but he did understand the Slytherin's tone. "What's that supposed to mean? Are you insulting Marguerite?!" he asked, his eyes narrowing angrily.
Standing on top of the Slytherin boy before he had a chance to stand up, Derry pointed his finger menacingly at him. "You watch your mouth when you talk to a lady, you hear me?" he said, his face going pink and hot with anger. Grabbing Selwyn's collar with his fist, he added, "Otherwise you'll have to hear from ME!"
Then, realizing he was probably talking way too loudly and that, surprise, there was a Professor in there midst, Derry let go of Selwyn. "Hehe, nevermind, uh..lost my cool there for a second," he said, patting at Selwyn's chest as though dusting him off. "Sorry Professor!" he shouted, his face now reddening in embarassment. "Look, he's okay," he added, petting Selwyn's head in the intention to look amicable.
Before taking a seat beside Marguerite, he placed his nose inches from Selwyn's and said: "Don't insult Marguerite again." He then strolled over to his desk, turned it into a huge bean bag chair, which he dropped down on happily. He smiled at Marguerite, rolling his eyes as he gestured towards Selwyn. "Ha, kids these days, right?" he said, laughing nervously.