Derek is ripping open a third sugar packet when he freezes at the intense rage that's suddenly, unarguably, being directed at him. He's bent over, vulnerable nape exposed, and his hackles raise as he slowly lifts his head and ducks his chin on instinct.

He's confused and he has no idea why Oliver is suddenly so angry at him, angry enough that Derek's lip curls back over his teeth in response before he schools his features.

But then Oliver says Thea's name so intensely, so possessively, and everything clicks. Oliver mentioned having a sister in the city, and Derek just slept with her.

"Your sister," Derek says regretfully, closing his eyes for just a moment before they snap back open. He keeps his voice as calm as he can, but it's low and strained. "I didn't know."

Derek's greatest talent has always been destroying things important to him without even trying, and his chest aches just a little to think that his burgeoning friendship with Oliver might now be just another thing to add to that ever-growing list.