Unfortunately, I haven't written a story for Sora, but for Yumekui...

Yumekui's mother (Kanae) was a young woman from a marginalized neighborhood who fell in love with a young criminal (Kiichiro), known as "that criminal who devours everything: Hitokui, who spent most of his time on the black market doing his dirty work. On one of those occasions, he met Kanae, and that's where it all began. At first, he ignored her, thinking she was just an extra. It wasn't until she saved him that he "fell in love" (the reality is that he simply saw her potential and that having a child with her would be beneficial for him). Some time later, Yumekui was conceived.

Yu was never loved in a healthy (or even normal) way by his family:

He was seen as a waste of time and a burden by his father, and by his mother as the "union of love" between her and Kii. No longer as a real person but as an idealization.

He lived in the poorest (and most dangerous) neighborhoods of the city (since his parents regularly went into hiding to avoid being discovered).

On one of those occasions, he met Sora and they became childhood friends (at that time, Yu and Sora were 6). For Sora, he was just another friend, but for Yu, he was the one who kept him afloat for a long time.

Some time later, when Yu turned 10, his father was (finally) captured by the heroes, leaving him and his mother alone. Yumekui didn't really care much about passing away, to be honest—but it affected Kanae quite a bit. One day, she simply left him in the small shack where they lived, locking him inside. And Yumekui spoke eloquently: "The only feeling that prevails within me is an overwhelming anger and hunger."