Nagu's past. He and his sister were abandoned by their parents, left to wander the desert in search of them. Sadly, his sister died before he gained control of his Hayagami. He later used it to resurrect her as a drawing and give them friends and a home.
Yorunami only ever wanted to reach his mother's expectations, but he could never meet them before she passed on. Then it turned out that she was only tough on him because she knew that his destiny as a shou would mean a very dangerous, difficult life.]]
Suguru's death. Harunawa says that he'll spare him if he agrees to say that he doesn't care about Hinohara, but Suguru refuses.
Kikitsune was originally a famous musician from Pompeii. Yes, thatPompeii. However, a jealous musician sent a slave to attack him, and the attack cost him his hearing. He got his revenge, but wound up pegged for murder.
Kagura's eternal youth. He found his Hayagami as a child, so he was forever cursed to never be able to grow up.
Also, it turns out that he was an amnesiac unable to remember his life before obtaining his Hayagami. This hits his only living family, Eto, really hard. And Kagura only remembers this right after Kadowaki forcefully absorbs Eto.
Yataka was in love with Himeou, but her role in society always kept them apart.
The flashback Arata has of Harunawa murdering his parents.
Arata and Co. eventually come across a man from Japan who's been stuck on Amawakuni for fifty years. He wound up traveling between the two worlds right before he was going to get married, too. And since the person he switched places with died, he can never go back.
Isora was once a kind and happy noble in Inquisition-era Spain who was betrayed by one of his friends, whom he'd saved from being burnt as a witch. The Bishop in charge of the Inquisition convinced his friend to throw Isora under the bus so he could get his inheritance. He was then tortured until he sold out his entire family to end the pain. Before being sentenced to the stake, his tongue was ripped out using a hot iron and he was thrown into a river. When he came out of the water, he was in Amawakuni.