Merlin was a good person character in the original Nanatsu no Taizai series. Now in it's sequel, Nanatsu no Taizai, mokushiroku no yonkishi as far as I've been teased, she's with Arthur Pendragon who is the antagonist.
Did Nanatsu no Taizai Merlin turn evil?
As of chapter 124, we know that Merlin left Camelot in disagreement with Arthur before the events of The Four Knights of the Apocalypse, and her deeds (or misdeeds) during the time skip have not been revealed yet. The person who's next to Arthur in his first appearences is not Merlin, but a captive fairy whom he has cast an illusion on and is having pose as her (chapter 91).
