Unmarked spoilers are ahead, per Spoilers Off rule.

Manga/Wish

Shuichiro's death, big time. Not only is it sudden and rather cruel for him to die with no apparent cause after everything seems to be coming together but he made a massive impact on every main character. Ruri and Hari are in tears, Koryu is angry and upset, Hisui and Kokyu are despondent, Shinichiro is also crying after having lost another family member so young, and the elemental angels all feel deeply sympathetic. And then there's Kohaku, the person who loved him the most, who had not only seen his death but was also still reeling from God's "punishment." It all becomes too much to bear so Kohaku chooses to be sealed in a tree with the others' help until Shuichiro is reborn again. The goodbyes everyone gives Kohaku also become this since everyone, even Koryu, has grown to truly care about the angel.