To access Angel (which can be opened after player character reach lvl 15) you have to open Angel option at the bottom side of your screen. (See the picture below)
Once opened, you will have 2 Angels: Shadow and Celeste.
From there, there are 3 major options to improve your Angel: Angel Promotion, Augment and Set (where you select what skills you want to use from the ones you have activated)
Each Angel has her own skills, so choose wisely which one you want to use. Here are the skills that each Angel has:
You can unlock all these skills through Augment. Rank up to activate numerous advantages!
Also the skills that you activate, you can use up to 4 in the same time. All you have to do is to rank up in Augment to Level 4, 6 and 8. The first skill you can use as soon as you unlocked any.
Angel Promotion will give you a great boost and power. So by enhancing your Angels your stats will improve, such as: HP, ATK, and DEF. When you Enhance, you will have a success rate that will not always be 100%. So you either risk or simply use Diamonds to make sure you will not fail.
And last, but not the least, Deploy the Angel that you wish to use! Happy hunting!