The Guardian Angel system in League of Angels allows players to utilize undeployed Angels. It becomes available at level 55. Get to it from the Angels menu.

Increase the stats of your character and heroes by dragging undeployed Angels into the slots next to them. 

The stats boosted are HP, PATK, PDEF, MATK, MDEF, and defense against enemy Angels.

  
The amount buffed is influenced by the Angel’s level and the slotted Cherubstone.

Cherubstones may be purchased from the Shop as well as the Arena Shop. Lower level Cherubstones may be synthed to create higher level ones.