Brain/Zero

Brain (also known as Zero) is a major antagonist in Fairy Tail, serving as the leader of the dark guild Oración Seis.

He has a dual personality:

• The "Brain" persona appears calm, intelligent, manipulative, and fame-seeking, presenting himself as the calculated leader.
• The far more dangerous and dominant "Zero" persona is sadistic, ruthless, and obsessed with pure destruction—he's the true master of the guild.

Brain/Zero's goal (particularly during the Oración Seis arc) was to acquire and unleash Nirvana, an extremely destructive ancient magic capable of turning light to darkness (and vice versa) on a massive scale.

He possesses powerful magic, including Dark Magic and the ability to use Genesis Zero (a devastating attack), and he has special markings on his face tied to the status of his guild members. In the story, Zero eventually takes full control, but Brain is ultimately defeated and killed (by Cobra in the original arc).

(Brief note: A version of Zero later reappears in the Fairy Tail: 100 Years Quest manga as a demon, but the classic Brain/Zero is from the original series' Oración Seis storyline.)