Oh foolish Mortals. I’m here to conquer not help.

Black visited Universe 6, went to the Super Dragon Balls from the unaltered main timeline within that universe, summoned Super Shenron from said timeline, wished to switch bodies with Goku from the unaltered main timeline's Universe 7, and Super Shenron granted Goku Black's wish. Goku Black sought to destroy all mortals alongside Future Zamasu, an alternate counterpart of him.[4]

When he arrived to Future Trunks' timeline, he initially referred to himself as Goku.[5] However, due to the notable style and color difference between Black and Goku's clothes, he was given the name "Goku Black" by Future Bulma. Thus, he took the name even when he revealed his own identity. His original appearance is the same as his future timeline counterpart, being an above-average height Kai of slim built and green skin, along with a white Mohawk. However, after killing his former master, Gowasu, he wore his matching set of green with gold chains Potara earrings. After Goku Black's wish to switch bodies with Goku was granted by Super Shenron from the unaltered main timeline, the Saiyan body underwent some physical changes. While Goku Black is still essentially identical, he has a slightly darker skin tone and more "defined" eyes, along with a somewhat leaner physique.

His hair is also a noticeably dark shade of black with only four front spikes and three back ones. While initially wearing his Supreme Kai attire, after his wish to switch bodies with the Saiyan was granted, he began wearing a grey dogi, a long-sleeved black undershirt with a turtleneck, a red sash, black pants and pointy white boots. He also gave the right Potara earring to his future counterpart while still wearing the other earring on his left ear. In the anime, he also wears a Time Ring on his right index finger, while in the manga, his counterpart is entrusted with wearing it. Before acting on his dark impulses, as Zamasu, he was a calm and patient individual, willing to listen to his teacher's words. However, he long harbored an utter hatred for mortals, perceiving them as a blasphemy and the gods' greatest mistake. He came to hate the gods for not doing anything about their "mistake". After giving in to his desires, Zamasu mercilessly killed his former master as his first step down a dark path. Despite his cruel, aggressive, and certainly unsympathetic actions, even to his own kind, Zamasu feels perfectly vindicated, truly seeing no evil in his actions. He scoured several timelines for proof that mortals are a corruption of existence that prevents it from becoming a true paradise.

He uses mortals as the scapegoat for his misdeeds, using Future Trunks' violation of time travel as justification for his and Future Zamasu's morally questionable goal of exterminating all life (similar to Android 13, who also uses mortal shortcomings to justify his carnage). He believes that humanity has only continued to exist for all this time because the gods are too prideful to acknowledge their failure in creating mortals. Also, rather than using the Super Dragon Balls from the unaltered main timeline within Universe 6 to wish for mortals to be exterminated, he wished to switch bodies with Goku from the unaltered main timeline within Universe 7, displaying his infatuation with the Saiyan. This suggests that he wanted to destroy mortals by himself, as he believed that it was his own responsibility as if he were the Supreme God. Goku Black retains his past self's politeness and is graceful during his first fight with Goku as Black. On the whole, however, he became much more emotional upon his wish to switch bodies with the Saiyan being granted. After becoming Goku Black, he gained many opposite characteristics to his host body's personality. Like his future counterpart, he is extremely sadistic, callous, bloodthirsty, and hypocritical. Despite believing that killing the vast majority of the Earthlings, including Bulma, to be "justice", he displays considerable blood lust. He delights in the anger of Goku from the main timeline within Universe 7 over him killing Chi-Chi and Goten from the unaltered main timeline within said universe. Goku Black showed brutality and ruthless savagery, such as when he coldly tortures his foes/targets, hoping to extend their suffering as much as possible. An example is when he fractured Future Trunks' arm and shot a barrage of ki blasts at his defenseless opponent. Somewhat of a masochist, Black revels in others inflicting pain on himself, both from his newfound passion for battle and personal drive to grow stronger from every attack that strikes his new body. This trait seems to have been drawn somewhat from Goku from said timeline within said universe. Somewhat unhinged, he occasionally bursts into deranged laughter, as shown when he fought Goku from the main timeline within said universe for the first time. He undergoes sudden shifts in mood, as seen when he explains his motivation for wiping out mortals.

While willing to kill regardless, he prefers and desires to fight strong opponents, even holding back his full power in order to further enjoy a fight, always keeping it just ahead of his opponent's. Black shares the determination of Goku from the unaltered main timeline within said universe, never letting up on a goal once he sets his mind on it, as he vowed to find Future Trunks after he escaped to the past. He also shares Goku's sense of general independence, wishing to grow stronger through his own personal effort rather than taking shortcuts, despite his entire existence as Goku Black being a shortcut to power - a distinct difference from his counterpart. Unlike Future Zamasu, who views immortality as an ideal trait worthy of a god, Black openly stated his disinterest in such a power. He is vain; when he first revealed his Super Saiyan Rosé form, he described it as beautiful, towering above "the whole universe", and stated to Goku that he should rejoice, as he had reached the height of beauty by his hand.

In the manga, Black's personality is drastically different from his anime counterpart. While Black in the anime always manages to stay calm and never lose his composure, even when losing a battle, Black in the manga is more prone to losing his composure the moment things do not go his way. When losing his battle against Vegeta from the main timeline within said universe, he breaks down in a fit of rage. The difference in their personality is also reflected in their contrasting fighting styles. Black, in the anime, performs attacks with finesse and flamboyance. Black, in the manga, prefers using brute force alone to overwhelm his enemies, taking a more aggressive fighting style. The difference in personalities is likely due to Black in the manga never meeting Goku personally prior to their encounter in the future timeline - he never had a personal connection with Goku.

This ultimately resulted in none of Goku's traits, such as composure when fighting or passion for battle. Similarly, his relationship with Future Zamasu in the manga was significantly different than in the anime. In the latter, he and Future Zamasu genuinely considered each other kindred allies. In the manga, Goku Black considered Future Zamasu to merely be a pawn, while Future Zamasu likewise considered Black contemptible for forsaking his divine body, for the sake of power. At one point, when they were on the ropes, they were even willing to turn on each other briefly. Black was also more sadistic in the manga, as evidenced when he not only proceeded to mortally wound the standard Gowasu when the latter attempted to talk sense into Goku Black, but also asked rhetorically while stabbing him whether Gowasu really thought it would have worked a "third time", referring to Gowasu's earlier attempts at talking him out of his plans.

Goku Black and Zamasu are also in a secret relationship yet they tell nobody about it. If anyone mentions Zamasu’s name followed by something negative he will snap and get incredibly mad at them, no matter who it is. If he sees a Zamasu variant however he will get incredibly sad and rush towards him to hug him with tears of happiness in his eyes as he has a breakdown from sheer joy.