He's originally shown to be a strict military leader who's primary concern was what is best for humanity. Unlike Eren he's logical, calm, serious, controlled and has no time for drama and fighting. According to a noble official he's a bastard of slave's blood.
He always did his best to help the Survey Corps against the nobles and later he was chosen as the judge for Eren's trial.
Later Zackly arrives at the King's Court to settle matters, bringing news that the monarchy's misdeeds have been exposed and that he has seized control of the Central Brigade. Under his authority, Erwin is set free and all military branches officially no longer answering to the Monarchy.He's officialy now the highest authority figure among humanity within the Walls, however, after Zackly assisted in the revolution against the monarchy he shows a completely different personality. He noted to Erwin that his only interest in participating was due to an intense and long-standing hatred of those in charge. He took great delight in seeing their fear when they fell from power. In direct contrast to the idea that he acts in the best interest of humanity, he told Erwin that he does not care if the revolution helps humanity or not. His only interest is his own desire to humiliate the monarchy, such that he set up a torture machine that fed one of them their own feces and called it a beautiful work of art. Ultimately, he was willing to admit that he values his own life more than humanity. Zackly gleefully torture and humiliate the arrested nobles and consider his own torture device the greatest work of art ever conceived.