I want to discuss about your theories and opinions about the cartoon series.
1. Blue is probably a daughter of Reina and Maliki, I think that because they are alike- Dark blue hair, similar traits, she is a teenager, so the age fits for Reina to be her mother. If it is not a case, Blue might be Reina's close friend, relative or an ally. But I still think that they are a mother and a daughter, and if that is the case than Maliki is Blue's father, because Maliki is the only man she spent time with while she was imprisoned.
Cassidy and Kubral are the true villains- I think that they could save Maliki, but they chose not to, because they knew that his daughter is special, so they decided to make her an angel and use her for their own purpuses in the future- I think they planned everything long before the became teachers of the golden school.
Adoptive mother and father of Raf were guardian angels of Angelie and Maliki- So they felt the responsibility to protect their dissapeared earthly one's daughter.
On the path of metamorphosis, angels and devils die in the end-and are reborn as humans. Maybe Tyco and Sai couldn't pass the final trial because it was deadly and they thought about ways to pass it. There was no other way, but to let it kill them, so they thought that that was the end and that they failed (I personally don't like this theory)
Keep in mind that these are only my theories and nothing of this is confirmed. I just thought about the reasons behind some events and came to my own conclusions which don't have to be true. Share your theories about the cartoon and the characters