Severus stared at him for a moment, his expression blank and unreadable, and then finally he stepped aside and held an arm out for Regulus to enter. "Fine," he said.
Not fine, really, but Severus found he did not quite have the spine to turn Regulus away at the moment. As much as he hated the feelings clawing against his ribcage, they would be manageable, surely. Perhaps Regulus would prove a distraction of sorts, something neutral and familiar on which Severus could focus and hopefully use as a hub of sorts to drag himself back under control.
Severus left the door open and stepped back toward the bedroom, anticipating that Regulus would eventually follow. He opened the bedroom door a bit too roughly and then paused, holding it ajar with his fingertips. Of all the other Slytherins, the other students in general really, Severus had always tolerated Regulus the best. He was not arrogant, or presumptuous. He was clever, but not tedious. They had similar values and similar goals for their futures. Perhaps that was the most important part of all--Severus and Regulus both knew precisely where their loyalties lay, and they knew what sacrifices had to be made to ensure that the future they both wanted to see on that horizon came to pass.