Story

Zamasu is the Kaio North of Universe 10 who is training to be the next Supreme Kai after Gowasu.
He's a serious individual with an extremely strong sense of justice, and that is exactly what makes him so dangerous; he honestly believes that all beings unworthy of existence must be wiped out.

Zamasu has a sparring match with Kibito, the right-hand man of the Supreme Kai of Universe 7. Upon witnessing the match, Gowasu admits that his pupil is a skilled fighter, but that his personality still needs a lot of work.

(Dragon Ball Super Chapter 17)

Gowasu then proceeds to use the Ring of Time to visit Planet Babari 1,000 years in the future. Upon seeing that the beings there still have not changed their violent ways, Zamasu takes action and cleaves a would-be assailant in two.

(Dragon Ball Super Chapter 17)

When told that a Supreme Kai must always keep a calm and gentle soul while watching over the universe, Zamasu, still unwavering in his belief that strength must be used to suppress evil, stubbornly responds, "Then why... Why have I been spending all this time and effort to achieve this power?"

(Dragon Ball Super Chapter 17)

Finding himself curious about the Saiyan Goku who possesses the power to rival a god, Zamasu watches a match between Universe 6's Hit and Universe 7's Goku on GodTube. It is here that he learns of the existence of the Super Dragon Balls, magical objects that are capable of granting any wish whatsoever.

(Dragon Ball Super Chapter 17)

In order to attain the power needed to defeat any god in his way, Zamasu devises a plan to use the Super Dragon Balls to switch bodies with the Saiyan! To that end, he visits the all-knowing Zuno to ascertain the whereabouts of the Super Dragon Balls.

(Dragon Ball Super Chapter 18)

Zuno sends a message to the Supreme Kai's Realm regarding Zamasu's plan, and once Beerus and the others hear about it, they're stunned to say the least! They also discover that he plans to travel to the near future to take over Goku's body, thus becoming Goku Black and then bringing the world to ruins.

(Dragon Ball Super Chapter 19)

Zamasu declares, "If my plan has already succeeded in the future, that makes it even more certain that I won't die now," and then forcibly attempts to kick off his wicked mission right then and there, only to find himself quickly destroyed by Beerus.

(Dragon Ball Super Chapter 19)

First Appearance

"Dragon Ball Super" Chapter 17

Trivia

Other versions of Zamasu exist in different timelines!

(From Jump Comics "Dragon Ball Super" Volume 3)

Parallel Timeline Zamasu
This is a Zamasu that was not destroyed by Beerus.
This Zamasu successfully switches bodies with Goku to become Goku Black. He then uses the Ring of Time that he stole from his former master Gowasu and travels to another timeline.

(Dragon Ball Super Chapter 14)

Zamasu From Yet Another Timeline
In this timeline, Earth is ravaged by the Androids, leaving Trunks as its sole defender.
Here, the Supreme Kai of Universe 7 sacrifices himself to prevent Majin Buu from being resurrected. As a result of this, his counterpart, the God of Destruction Beerus, is also eliminated.
Goku Black arrives in this timeline, and upon seeing that there is no one capable of stopping him, carries out The Zero Mortal Project.

(Dragon Ball Super Chapter 19)

Trunks tenaciously fights back against Goku Black and even manages to injure him, but Zamasu appears and uses his restorative powers to heal away the damage.
Even worse, every injury that Zamasu heals triggers Goku Black's Saiyan cells and gives him a dramatic power up each time.

(Dragon Ball Super Chapter 20)

Goku uses Bulma's time machine to travel from the past in his parallel timeline to the future to challenge Zamasu, but even the mighty Saiyan warrior is unable to inflict any lasting damage to the fallen Supreme Kai. It is later discovered that this is due to Zamasu using the Dragon Balls to wish for immortality.

(Dragon Ball Super Chapter 22)

In a last-ditch effort to topple this diabolical, self righteous force of destruction, Goku unleashes the Mafu-ba to seal him away. While it seems to have worked at first, Goku had mistakenly used the wrong seal for the technique, which allows Zamasu to barely escape. Despite being unable to inflict any lasting damage to Zamasu, Goku gradually frustrates his foe with a ceaseless barrage of attacks, causing Zamasu to fuse with Goku Black using the Potara.

(Dragon Ball Super Chapter 22)

(Dragon Ball Super Chapter 22)

→Scroll down to the comments for a hint about who next week's character will be!

*All comic images are from the Japanese version.