The Unknown Melody (知られざる調べ Shira rezaru shirabe?) is an extra chapter of the Attack on Titan: No Regrets manga, written by Gun Snark and illustrated by Hikaru Suruga. It was included at the end of the second volume of the Complete Color Edition, after Side Story 4.
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.
Summary
Isabel recalls a conversation with an old beggar in the Underground city who taught her to sing, which had Furlan wonder about her intentions afterwards.
In the present, Flagon shows his squad the new formation for expeditions outside the Walls. This bothers Isabel, who feels sleepy and hungry, and Levi and Furlan notice it. Remembering the old man, Isabel begins to sing loudly, much to her squad's annoyance. Levi tells Flagon to get Isabel something to eat in order to silence her.
In the Underground City, a man talks to the beggar about Isabel. He complains about her terrible singing skills but adds he somehow misses her, which makes the old man smile.