Zeref

Zeref - at one time - was considered the most powerful and evil mage in all of a world's history. He held a very powerful and vicious magic capable to both create and destroy life itself. However, Nero has since killed the mage who once upon a time was a good friend to him.

Appearance
Zeref appeared to be a young man of about 18 years, even though he was already over 400 years old when Nero killed him. He was slightly taller than Nero himself and was somewhat melancholic by nature, his consciousness haunted by all of the terror he caused in the past.

Personality
Zeref used to be a kind and caring person who would use his magic to revitalize dragons defeated by Acnologia to spread the idea that humans and dragons can be equals. However, once he started going down the wrong path he became a cold and evil person who lost his care for life, however sometime after he battled Nero and Natsu of Fairy Tail he began to value life again and went to live in seclusion, which ended his reign of terror for the time being.

Centuries later Zeref had become a sorrowful man who wished to repent for his sins. He rarely became angry, only doing so at the Dark Guild Grimoire Heart when they wished to use him to take more lives. When Nero confronted him for the last time and helped free his heart and soul from the evil that gripped it, he thanked Nero sincerely for allowing him to die in peace.

History
Originally, Zeref and Acnologia worked alongside Nero's group in the Dragon Civil War. Acnologia would be the major powerhouse in defeating dragons, and Zeref would heal them afterward in attempt to show that they were not monsters or trash but that they could be equals.

However that all changed as the two delved too deep into their magic. Zeref began conjuring demons to do his bidding. He summoned more and more powerful ones - even creating a few of his own - but even he was eventually tricked. A demon feigned its submission to him and eventually wormed its way into his heart and took control of his magic. If Zeref cared about life, the demon would ensure his magic activated without his control. But if he did not care whether the target lived or died, he would be in full control. It ensured that Zeref would always kill so that the demon could drag the souls to the underworld.

Eventually Zeref fell into the demon's spiral and began killing many people and creating many of his own demons. He even discovered the way to turn Acnologia into a true dragon. After this event they were confronted by Nero and co. but ultimately defeated them. Nero's last-ditch effort to save Zeref involved using Kotoamatsukami to ensure that he would not kill anyone anymore (not knowing at the time that his magic would be beyond his control).

More than 400 years after this event, Zeref only wished for his death to atone for everything that he did. However after being confronted by Nero yet again, he was treated with respect until his end. Using his Empath abilities Nero entered Zeref's heart and helped him confront the demon, eventually agreeing to give up the part of his spirit that allows him to form magic power in exchange for his freedom. The "physical" soul left behind was allowed to die peacefully, and Zeref thanked Nero for being the one to kill him in the end. Zeref's death spurred Nero's summoning of his "Persona of Grief", Kokuel. It is implied that this Persona at least partially symbolizes Zeref himself, as the remnant of Zeref's spirit summoned Kokuel to attack White Nero during the "Characters Chapter".

Quotes

 * "Thank you, Nero... ... This is what I wanted, and I'm happy to have been killed by you in the end. You are a true friend, so never forget your kindness. Thank you, and good bye..."