The Unknown Melody

Levi's Unknown Melody?! is an extra chapter included in the Complete Color Edition of the Attack on Titan: No Regrets manga, written by Gun Snark and illustrated by Hikaru Suruga.

Overview
Isabel is taught by a man on the street how to "sing." She later puts this skill to use when she is hungry, causing Furlan and Levi to state that she does not shut up until she is fed.