"Why not her? She asked for my help, I helped her free of charge, now I'm bored and I want her." Mephistopheles wasn't going to tell his full reasons to the pawn, that would be silly.
"If you'd rather not see Jenny again, I'm perfectly capable of doing it myself," Mephisto said dismissively, grabbing the wrist of the serving girl, tracing his sharp nails over her jaw and neck.