先生ごめんなさい

A young couple, Yuzu and Kiyoshi, and Kiyoshi's father, Junichi, who is suffering from the aftereffects of an accident, moved into a house in the countryside. When they entered the attic, they noticed a presence. A boy by the name of Hotaru had taken up residence in the attic. What is the identity of this mysterious boy? And what is the hidden past of Yuzu and her father-in-law, Junichi? (Source: Futabasha, translated)