Zamasu

Zamasu is a character from Dragon Ball series, he appears in Dragon Ball Super as the main antagonist of his own arc.

Background
Zamasu was formerly the North Kai of Universe 10, and was a witness of the countless wars done by mortals. He was chosen by Gowasu to be his successor, based on his battle talent. Zamasu was happy about it, and promised to do his best for universal peace.

Zamasu Saga
In the anime, Zamasu is first seen carrying a cart of refreshments. He then gives a cup of tea to his master, Gowasu. His master then tells him about his duties as a Supreme Kai, which is watching over their universe including mortals. Zamasu then asks him if the mortals are worth their protection, due to their past mistakes. Gowasu explains that the Supreme Kais' and Kais' duties are to create and not interfere with creations. Zamasu implores if Gowasu is telling him to look past the mistakes of mortals with Gowasu simply telling him to know his place, watch over and raise them. Zamasu is silent and reluctantly accepts the situation when Gowasu repeatedly implores for a response from him. Beerus, Whis, and Goku then show up with the latter repeatedly asking Zamasu to fight him, but is then stopped by Beerus. They then tell them about someone who is similar to Goku and how he has a Time Ring. Beerus asks to see the Time Rings. Shortly afterwards, Zamasu reluctantly fights Goku after Gowasu instructs him to do so, stating that it's part of Zamasu's training. Goku turns into a Super Saiyan 2, much to Gowasu's and Zamasu's surprise. During their battle Goku, Whis, and Beerus feel Zamasu's ki, saying it is very similar to Black's. Goku then overpowers Zamasu, defeating him in battle. Goku offers his hand but Zamasu reacts with hostility before being reprimanded by Gowasu. After they leave, the fight only furthers Zamasu's hatred for mortals, now thinking they are dangerous. However, Goku notes his ki is not quite the same as Black's. However, Whis is convinced that Zamasu and Black are in fact the same person not only due to their ki but the fact that he felt the same coldness in his heart that he felt from Black.

In the manga, Zamasu goes with Gowasu to meet the Shin, the Supreme Kai of Universe 7, and spars with Kibito, he appears to be about to end the battle too violently and so Gowasu stops him. Zamasu is then intrigued to hear about how a mortal such as Goku was able to defeat the likes of Majin Buu - thus having power greater than a god.

Zamasu and Gowasu become aware of creatures on the planet Babari while sitting for tea. Zamasu advocates for the destruction of the creatures, only to be rebuked by Gowasu who asserts that it is the place of the God of Destruction to carry out that act and not his. Gowasu then shows Zamasu the Time Ring, explaining its function to him and Zamasu also learning that one cannot travel back to the past with them. Gowasu prepares for the two to depart to the future of Babari though mentions that Zamasu cannot go since he is only an apprentice and not a Supreme Kai, before passing Zamasu an earring and allowing him to be one for the time being. Zamasu puts the earring on the ear corresponding to Gowasu's to ensure the pair do not fuse and denies interest in merging with him for the time being. The two fist bump and say "1000 years", transporting into the future of Babari where a fight breaks out between the planet's denizens, Zamasu concluding their continued existence would ensure no peace.

Observing the Babarians with Gowasu, Zamasu views them as accomplishing goals with brute force and a disregard for laws or justice. A Babari attacks them, Zamasu repelling it a short distance away from them with his hand and goes against Gowasu's wishes by slicing the creature in half, killing it. Returning to their planet, Zamasu returns the earring to Gowasu upon the latter's request and later prepares tea, rebuking his mentor's claim that they can learn from mortals, instead reasoning that they cannot learn given the example the Babari set. Gowasu tries explaining justice is meant to counteract evil but Zamasu forwards that mortals have used knowledge to dirty their world and that the ones with knowledge are evil. Gowasu warns that he must calm himself and that the Gods should also learn, Zamasu leaving to fetch Gowasu more tea and thinking non-interventionism is a crime.

Later, while watching the fight of Universe 6 and 7 tournament on God Tube with Gowasu, Zamasu sees the Super Dragon Balls. Zamasu, curious to why the Gods of Destruction would be fighting over the Super Dragon Balls, asks Gowasu what they are. Gowasu explains to Zamasu that they are orbs that can grant any wish, much to Zamasu's surprise. Also while watching the tournament, Zamasu sees Goku covered in godly Ki and much to his anger as he wonders how mere mortals could achieve this kind of power. He heads to Zuno's place and asked them to know more information about the Super Dragon Balls and Goku, which a flustered Zuno complies to.

Later in the anime version, Zamasu re-appears in Universe 10 with Gowasu's tea, apparently calm despite the appearance of Goku, Beerus, Whis and Shin. Zamasu apparently manages to see off his guests and reunites with Gowasu. After confirming their suspicions of Zamasu's true intentions, Goku, Shin, Beerus and Whis head out to protect Gowasu and make him realize Zamasu's currently evil nature. Though shocked by how much they know about what he thought and planned, he is undeterred and attempts to attack Goku with an energy blade. However, Beerus intercepts him and erases his existence afterwards, putting an almost instantaneous end to the once prodigious Supreme Kai apprentice. Gowasu speaks his name as he seemingly laments that his apprentice had to pay a price to learn of his wrong ways. In the manga version, Beerus and Whis head to see Zamasu and Gowasu without Goku or Shin.

Original Future
In his original future, Zamasu went on to become Goku Black and terrorize Future Trunks' timeline. This future was visited by Shin in the manga to confirm Zamasu's evil.

Due to Beerus not being informed of Zamasu's future events and thus not destroying him, Zamasu succeeded in carrying out his plans to kill Gowasu and take his Potara Earrings and Time Ring. Following this, he used these items to gather and use the Super Dragon Balls to switch his body with Goku and killed the Goku in Zamasu's body, Chi-Chi and Goten; and traveled to Future Trunks' timeline and allied with his future counterpart there.

This Zamasu then initiated the main stage of his Zero Mortals Plan, terminating the Supreme Kais, therefore the Gods of Destruction, annihilated several mortal planets in the Universes, and destroyed Earth's civilization, becoming known as Goku Black. Black fought Future Trunks numerous times, aiming to gain full control over Goku's body. Eventually, Future Trunks fled back in time to get help from the heroes of the past.

Goku and Vegeta return to the future with Future Trunks and Vegeta is the one to battle him, Black uses his Super Saiyan form to battle Super Saiyan 2 Vegeta, but proves to be outmatched., Vegeta turns into Super Saiyan Blue and tells Black to use the form too, but Black reveals that he cannot and so he is beaten back by Vegeta. At the same time as Super Saiyan Blue Vegeta's initial battle with Super Saiyan Goku Black is coming to a close in Future Trunks' timeline, Beerus goes to Universe 10 and destroys Zamasu; splitting the timeline - thus erasing the original future from happening and creating a split timeline.

However, the Zamasu who had succeeded in the original present timeline was still alive and carried on, eventually learning of his past self's death from Goku and Vegeta in their first trip to the future in the manga. Ultimately however, this Zamasu met the same fate as Present Zamasu, erased from existence - albeit from Future Zeno.

Dragon Ball Xenoverse 3
Zamasu appears as a playable character.

Dragon Ball Z: Invasion From Hell
Zamasu is one of the Sub-Bosses of the game.

Trivia

 * The word "Zamasu" may have come from the Japanese verb ざます(Zamasu) which translates as "to be, to exist". It shares exactly the same definition with his former master's hypothetical name etymology, Gowasu.
 * Zamasu is the second person to be promoted from the position of Kai to the position of Supreme Kai, although he was only Supreme Kai temporarily.
 * Zamasu was shown to have a pet of his own as a Kai; a small winged pig, similar to King Kai, with his pet monkey Bubbles. Interestingly both Zamasu and King Kai were their Universes North Kais.
 * Zamasu's outfit in the anime shares the same color scheme as Shin's in the manga.