Cygni, the Angel of the Stars, is one of the four Angel sisters who serve Binary.
She and her sisters work to keep the Celestial Powers at bay, and keep the universe in balance so that Cthulhu never awakes again. They have dealt with the Moon Lord, Cthulhu's brother, multiple times when he attempted to upset their hard won balance.
Cygni is the Summoner, and relies upon forming the stars' essence into physical embodiements of the Astral Guardians. She can summon any creature known, and has a wider ability to summon objects than her sisters.