The Sun Summoner
There has only been one Sun Summoner, Alina Starkov.
She has the ability to bend and summon light, but needs there to be light existing first. In Shadow and Bone, it is revealed that when she was tested as a child for signs of her Grisha ability, she knew that she was a Grisha, and that she was different from Mal. When she realized this, she hid her power so that she would not be taken away from the only good thing in her world. However, she did not realize that she had ever been hiding, or had ever had had the power, until she saved Mal's life in the Shadow Fold.
Because she was the only one of her kind, the Darkling felt a particular connection to her, as he was also the only one of with his power, and believed he needed her to balance out his darkness.
Alina Starkov is classified as an Etherealki, as is the Darkling.
However, at the end of Ruin and Rising, after using all three of Morozova's Amplifiers, her power transcends herself through said amplifiers and is taken from her. Instead, the Sun Summoning power is bestowed upon random people around the country, perhaps further. Alina loses her powers as a Grisha, passing it onto the others in her stead and it is these new Sun Summoners who go on to destroy the Shadow Fold. It is currently unknown what becomes of these newly created Grisha and their gifts.