Gyuki, born Umewakamaru's past, at the age of seven, after his father's death, he began serving at the temple on Mt. Hiei to pray for his father's soul, and it was during this time that his mother left him there and went off on her own. Though Umewakamaru showed a great aptitude for studies and learning, his peers at the temple became jealous and injured him several times. Realizing he didn't belong there and missing his mother, he snuck out of the temple and began making his way to the capital. At Ootsu, he was tricked by a pair of yokai who claimed that his mother was ill and staying with them on Mt. Nejireme. As a result, he was eaten by the same yokai which had devoured his mother. However, through the power of his hatred and will, he was reborn as a yokai after his death - destroying the yōkai which had killed his mother and himself in the process. That yokai was the previous Gyuki, and over time the yokai who had once been Umewakamaru became known by the same name.
Hihi's death at the hands of Muchi, a very sudden and undignified ambush, showing that the Nura Clan's decline in power that it used to have. When Nurarihyon meets up with Muchi, he recounts how he and Hihi were drinking buddies and then goes into Tranquil Fury when he kills him.
Shoei's reaction to this is equally saddening, especially with later chapters show how good of a father Hihi was to him. He can barely contain himself when Tamazuki is cornered, desiring revenge on the man who led his father's murderer.
Yamabuki Otome's failed relationship with Rihan due to be being infertile because of Hagoromo Gitsune's curse. She leaves him, only to be used in a complex and cruel scheme by the Nue and Sanmoto in order to get revenge on their mutual enemy, Rihan, brainwashing her to kill off her lover and the following anguish of this act.
Abe no Seimei sending Hagoromo Gitsune to hell, despite her centuries long determination to birth him, she is genuinely shocked when he betrays and his callousness sells it well.