It's a little flattering that Isaac seems to be interested in his stupid scribbling, and he makes a little noise in his throat.

"Wait, what, my back?"

That's a bit confusing, and if Klaus isn't reading things wrong, maybe a little kinky? Just a little. Smirking, he turns to look at Isaac.

"Not sure how I feel about a portrait of me on my own back. Seems a little uh, narcissistic, right?"