In the modern era, Zeref seeks to isolate himself and is very calm, sincere, and conflicted. He wishes to die to atone for all the sins he has committed, and may commit in the future. He shows concern for others when he warns them not to come near him, knowing that they would die if they did. He is also somewhat emotionally fragile, as when he realizes that Natsu is not ready to kill him yet, he breaks down in tears. Zeref often kills with no prior incentive or knowledge.
In spite of all this, Zeref is very vindictive, and will punish those who he believes to be evil. This was shown when he ruthlessly killed Hades, whom he thinks has committed a number of unforgivable sins. As a result of his immortal life, Zeref has developed a misanthropic loathing for humanity and its repeated idiosyncrasies of hatred, evil, and conflict.
Nevertheless, Zeref is also shown to be a man of the people as the Emperor of Alvarez, and is shown to display good will and friendliness towards his generals and subjects, which is also reflected in their opinion of him, sincerely cheering and welcoming him as he returns to the capital after a long absence. Zeref also compared running the Empire akin to playing a strategy game, making the Curse of Contradictions' symptoms go away while at it, because the people he commands, he treats merely as pawns.
Orphaned after Dragons killed his parents and his little brother, Natsu, an extremely young Zeref became a student at the Mildian Magic Academy, where he researched the connections between life, death and Magic to revive his brother. After creating things such as the R-System and the Eclipse Gate, he was expelled from the academy for breaching the sacred teachings of Ankhseram, however he was cursed by Ankhseram and then killed everyone around him with his newfound, unwanted Curse of Contradiction. After this, Zeref wished to die and wandered around, creating the Etherious in the hopes that they would kill him, but instead turned to reviving his dead brother, having preserved his body, as an Etherious, thus fulfilling his goal of bringing his brother back and creating something that could kill him: E.N.D.; Etherious Natsu Dragneel.
One day while out gathering herbs, Zeref stumbled across the Fire Dragon King Igneel, who unlike many other Dragons at the time was not opposed to associating with humans, and after acquainting with one another, the Dragon agreed to teach Natsu Dragon Slayer Magic and raise him. Zeref then became aware of the plan hatched by Igneel, Metalicana, Grandeeney, Weisslogia and Skiadrum to use the Dragon Soul Technique to seal their damaged souls inside young Dragon Slayers that they have raised and travel to the future, where the Ethernano-dense air would help them recover enough to kill Acnologia. Zeref helped them do so, and had the Celestial Spirit Mage Anna Heartfilia open the Eclipse Gate to the future, while also four hundred years later having Layla Heartfilia open another version of the Gate to connect the two time periods, allowing the Dragon Slayers chosen (Natsu, Gajeel, Wendy, Sting and Rogue) to arrive in the future on July 7, X777.
Ultimately, the true tale of events became distorted with time, resulting in legends surrounding the now-dubbed Black Wizard. The stories stated that at some point in the distant past, around 400 years ago, Zeref witnessed the deaths of many people. Around this time, he also became cursed, and unintentionally took the lives of all those around him. Zeref was described as having alighted in an ancient land, mastered Black Magic, given birth to tens of thousands of Demons, and thrown the world into chaos.
At least a century ago, Zeref founded what later shaped itself to become the Alvarez Empire, which united 730 guilds of Alakitasia by martial force, but he rarely dabbles in politics unless necessary, preferring to stay beyond the capital, or even the empire itself, while his ministers and generals do the majority of the work.