Acnologia

There was a sense of someone's terrible presence in the depths of the cave.

(To Igneel) "To think that Dragons still live... I am displeased."
(To Igneel) "I recognize you as my enemy, Dragon. And I shall slay you."

Personality

Four hundred years ago, Acnologia was a ruthless, bloodthirsty, and violent person. He killed any and all Dragons he could, ignoring the fact that many were his comrades.

In the present, Acnologia is a recluse who wants nothing to do with humans, in spite of the fact that he is one. As Zeref Dragneel puts it, he sees other humans as nothing more than insects, ignoring any and all attempts by them to converse, which suggests that Acnologia is highly arrogant. However, inversely, Acnologia's motives are also shrouded in mystery, as even with one arm he, according to Zeref, has the power to rule the world yet elects not to do so. Insights into his wishes have only been touched upon by outside sources, which include his vague desire to fight against humanity, as well as face an opponent strong enough to give him a real challenge. Ultimately, in a form of irony, despite his inherent hatred for humanity, Acnologia prefers to take on the shape of his old human form in casual instances

While in his Dragon form he has shown to be very destructive and violent as described before in legend, in his human form, Acnologia retains a stoic and composed personality, even after striking down God Serena, Acnologia still maintained his stoic composure. However Acnologia possesses a love for battle, as Zeref theorized he was looking for a worthy challenge and let out a smile after Zeref promised him he will soon have a worthy fight. When Acnologia comes across strong opponents, he shows a more excited side to himself, such as during his fight with Irene Belserion he smiled with excitement and anticipation after witnessing the power of the Shield of Spriggan, even giving her praise.
Like with most Dragon Slayers, Acnologia is still susceptible to motion sickness even while in Dragon form.