"Ugh." Regulus prides himself on keeping his emotions off his face, but his disgust at the idea is transparent. Bertie is almost worse than Sherbet Lemon. "I'd really rather not. Why don't you just call me..."

He glances sidelong at Potter again, and his thumb presses hard against the glowing mark on his arm, the phantom itch of it.

"Mr Black," he says, at last. It's not much of an alias. A Potter will undoubtedly know the House of Black, and nobody's likely to mistake him for Sirius - they may have looked alike as children, but between Sirius' rebellion and Regulus' determination to not turn out like his wild brother, that resemblance is long gone. Still, somehow it's easier to maintain at least a veneer of plausible deniability. "You may as well call me Mr Black."

Watching Potter sidelong as he says it, biting down on the inside of his cheek as he waits to see whether there will be a reaction.