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Grisha are the mages of the "Shadow and Bone". They perform Small Science. There are 3 orders: Etherealki or Summoners, controlling wind, fire or water; Corporalki, manipulating human body, healing or killing and Materialki, the creators of materials and explosives.

You can read more here on the Grishaverse website. It also has 2 quizes (I took one of them and got Alkemi, that's why I picked them to make a skin of).

It's not any particular character, just a person wearing Alkemi kefta.