In the Zero series (developed by Inti Creates), he was the protagonist and had a change in his design that was meant to impart a more "human feel" to him.

[3] Zero debuts as a fully playable character in Mega Man X4, where he fights the rebellious Repliforce and is forced to kill an old friend in the process, Iris.

[4] In his scenario, Zero is haunted by nightmares of a shadowy figure awakening him and giving him orders to destroy an unknown individual, and visions of ensuing carnage.

Set around 100 years after the X series,[10] an amnesiac Zero awakens from stasis and helps a scientist named Ciel fight the human city of Neo Arcadia, which is persecuting the Reploid race over an energy crisis.

Zero eventually convenes with the spirit of X, now a Cyber-Elf, and fights the latter's doppelgänger Copy X, the Four Guardians of Neo Arcadia, and ultimately Dr. Weil and his companion Omega.

Zero also reprises his role from the first game in three mangas by Iwamoto Yoshihiro,[12][13][14] and the prequel Irregular Maverick Hunter X by Ikehara Shigeto.

When asked if Zero had killed the cast of the original Mega Man titles, suspected due to their absence in the X series, he replied no, adding that given how he had designed the character, "Zero is not such a person--it is not in his profile.

In order to make Zero's story more engaging, Capcom created Iris, a female Reploid, believing it was a fresh idea not used in previous games in the series.

[35] Zero's depiction in the series was intended to be morally ambiguous and depend on the perspective, appearing as a hero from one point of view and a terrorist from another.

[38] Based on his points of view from the original Mega Man X, Higurashi wanted fans to see X and Zero as dark archetypes of heroes rather than typical ones.

This was found challenging to animate in Mega Man X4 as the pixels are meant to be divided between his red armor and long blond hair.

Zero also started wielding his signature weapon, the Z-Saber, an energy-based sword that introduced melee combat to the Mega Man games.

His mecha-inspired armor was eschewed for a red uniform consisting of a vest, gloves, and boots, while his arms and legs were more anatomically detailed.

[52] GameSpot editors Christian Nutt and Justin Speer X3 benefited from the addition of Zero as a limited playable character as previous games from the franchise only used both Mega Man and X.

[53] In regards to Zero's powers, Game Informer claimed Zero had the best weaponry in X6 based on the multiple extra abilities he had before defeating any boss character.

[54] Jeremy Parish from 1UP.com stated that his appearance as a playable character with his own story in Mega Man X4 by itself made it the best game in the X series.

[57] GamesRadar also cited Zero's actors across the X series, finding the first from X4, Wayne Doster, unappealing due to his infamous scene where he yells in front of the dead Iris.

[58] While reviewing Mega Man X: Command Mission, 1UP.com criticized that his English voice acting makes him "sound like a surfer" and lamented his poor screentime.

[60] IGN repeated their positive sentiments about Zero in their list of characters they wished to see appear in a future Marvel vs. Capcom title, describing him as "arguably cooler than Mega Man", regardless of version in comparison.

[61] PSM praised the character as well, stating "[he] might wear some funky shoes, but that doesn't stop him from kicking some robot butt".

[65] In regards to his Mega Man Zero incarnation, GameSpot believed the character's redesign was well employed based on his presentation.

[66] IGN considered Zero's retake be like a breakout character due to how he develops his own skills unlike previous versions of the Mega Man cast.

[67] Destructoid simply described Zero as "a red death machine who uses pistols, lightsabers, shields, and multiple other melee weapons..." due to his dark characterization when compared with previous main characters.

The writer further added that the apparent misrelationship between Zero and X before the revelation that the latter was a clone gone corrupted was one of the darkest things seen in Capcom's games due to the idea of Zero working to kill X, the former protagonist of the series.

As it is believed that Dr. Wily created Zero in the image of Protoman or the possibility that he was responsible for multiple deaths offscreen as it is unknown what happened to the original Mega Man cast, explaining their absence in the X storyline.

In 2018, Capcom left clues about how Wily dies when creating Zero but the idea of Zero killing the Mega Man characters was negated.

[73] In a comparison of the three main characters from Mega Man X8, Luthfie Arguby Purnomo from Studies on Shift noted that X's weapons symbolized his status as a cultural hybrid in contrast to the Western Axl and the Eastern Zero.