In the latest chapters, it is finally revealed that Yuno is from a royal lineage of the family that ruled the Spade kingdom. He was left at the church due to an attack on his family — the House of Grinberryall.

Why were ASTA and yuno abandoned at the same time?

About 4-5 generations later Asta was born but his parents were forced to abandon him around the same time as Yuno to protect him.

What happened to Yuno?

Yuno gets back up and fatally stabs the Fifth and his Future Diary. However, after being defeated, Reisuke gives Yuno a warning about her future with Yukiteru: there can be only one survivor, and they will have to eventually try to kill each other.