In a future chapter of petite reprise, killer bonds with robin as she reads her stories aloud for him and kidd as nami paints thier nails.
I see him as an information sponge from a young age, and robin is full of new things he's never heard about. And while it didn't quite make it to this sketch, her extra arms dont bother him at all and he rather likes holding her hand even if its not attached to her at the moment.