Angels are pure and innocent beings. 

They were once human who died at a young age due to unfair circumstances or while saving other people. Only pure, innocent and good people can become Angels after their death. Every Angel has his/her own past.

The Angels messenger went to them after their death and lead them to God where they recieved their powers before being sent to Heaven.

Angels have only one power which is  based on their past human's qualities or likes.

But Angels also have a weakness which is based on something bad that happened when they were human or simply on something they didn't like.

Angels can't breed with other Angels. Relationships between Angels are highly disapproved by God however, God decided that love shouldn't be stopped if it is pure and sincere. That's why he charged the Angels messenger to be the judge in those case. When two Angels fall in love with each other, they have to ask to talk to the Angels messenger who will hear the feelings of both Angels and decide wether those feelings are sincere.

The only mission of Angels is to found their 'protégé' who is the one they have to protect who is most of the time their soulmate. A 'protégé' is a person who has a lot of problems or is troubled by something that happens and scarred him for life. The role of the Angels is to make them open up and be themselves freely again.

Angels wings are a light grey but a grown  up Angel has white wings. To get those precious white wings Angels will have to lose their purity (ity) to gain another one (purity of love). Angels can't fly till their wings turn white.

Angels and Demons never met before. Angels are scared of them due to their dark Aura and every bad things people say about them. Which means that nobody knows what Angel/Demon couples would be like...

Angels can come and go freely in the human world but are absolutely forbidden to enter Hell.