Merlin

She is a member of the Seven Deadly Sins. She has the mark of the boars sin of gluttony. She was regarded as the greatest mage in Britannia and is the master of Vivian as well as a teacher of King Arthur of Camelot. She came to the aid of the sins in their battle against Hendirkson but had to leave to tend to the King. She has not officially rejoined the sins but said she would as soon as she was finished with her other business.

[Spoiler] The day when the Seven Deadly Sins was framed was when Merlin stole most of Meliodas' power, which he cannot control easily. [/Spoiler]