Rukifellth plays acoustic guitar. As a young boy, he'd shown no interest in the more "refined" instruments of his social class such as piano, flute, or violin, but he loved music nonetheless, so he snitched money from the family vault to buy his own guitar. His father, tellingly, was more unhappy with Rukifellth's choice of such a "common" instrument than about the stolen money, but Rukifellth's mother managed to convince him — not long before she died — to let Rukifellth have his guitar if that meant he could express whatever musical talent he had.
He doesn't actually have a lot of musical talent to express. But he's found that he apparently has enough talent to entertain Lilith with, and that makes him happy.
Lilith styles hair. It had been one of the sole pleasures afforded to her while in the Azurite Circle, under the pretense of evading recognition and subsequent capture by authorities. Curls, pigtails, odangos, mohawks...nothing is off-limits to her. She happily experiments with clips, bows, braids, dyes, and barrettes. Sometimes she even experiments with all of them at the same time.
And when she does, it's usually on Rukifellth.