Besides magic, Regulus' passion has always been drawing.
His mother Walburga never found any inconvenience in his past time when he was a child. Art was a noble past time and Regulus was really good at it.
So Regulus doesn't really know why he hid from her that he took drawing lessons every Saturday at Hogsmeade. Was it because his teacher was a half-blood? Or because he was afraid of criticism? Whatever, it was his little secret. He could have one, right ? Sirius had plenty of them, why not him?
That Saturday had been a bit special, his drawing teacher was sick, so he couldn't teach. But since Regulus needed the easel and oil paints to finish his painting, he had allowed him to take the keys and use the art studio.
That's why the Black hadn't expected to see you in the room. You were drawing in charcoal on your sketchbook the human skull in front of you. A muggle record was spinning on the turntable, the soft music and your humming filling the room. Your hand moved fluidly and delicately on the paper.
Regulus should have been disgusted by the music and your muggle clothes, as he normally would have been. Surprisingly, this time, he didn't seem to notice the fact that you didn't seem to belong to the wizarding world. Silently he approached you.