Merlin is a mage character from the Fate/Grand Order universe and anime. He is a half-human, half-incubi Caster-class Servant who was summoned by Ritsuka Fujimaru in the Grand Orders of Fate/Grand Order. He holds the position of Grand Caster, and is also known as "The Magus of Flowers".
In the anime, Merlin is described as one of the greatest mages of the world, and the one who helped herald King Arthur himself. Having been summoned by King Gilgamesh to help with the threat in Uruk. He travels with Ana and aids Ritsuka Fujimaru in their battle in Babylonia.
He has a rather joyful personality and is rather aloof and makes extravagant words towards himself. He is considered a scoundrel by many, and those like Ana and Roman consider him very annoying with even Fou attacking him. Of course while he doesn't hate humans, he is rather indifferent towards them and states himself he consumes human dreams to produce the human emotions he himself lacks.
Still, while 'inhuman' he doesn't consider this a bad thing at all. Enjoying the tales humans have and doesn't mind helping them along the way. In battle he will even get serious if it comes down to it.
He also likes to consider himself a 'Big Brother' to Mash and Ritsuka.
In the overall franchise, Merlin is described as a prophet and advisor who made his entry into Arthurian Legends as the court magus, he would at times guide, at times torment, and at times help King Arthur as a king so that she could overcome many enemies and hardships. Still, though he is a wise man, he is fundamentally inhuman, because he was born of mixed blood from the union of an incubus and a human. With illusions as his forte, he can transform into various shapes——there was a case where he had changed into the shape of a beast whose insides are not human——and his true form is not certain.