From Viz:
Chakuro is the archivist for the Mud Whale, diligently chronicling the lives and deaths of his people. As one of the short-lived thymia wielders, he knows his time is limited and is determined to leave a better record than his predecessors. But the steady pace of their isolated existence on the Mud Whale is abruptly shattered when a scouting party discovers a mysterious young girl who seems to know more about their home than they do.

Note: Nominated for the Tezuka Osamu Cultural Prize in 2017.