When the 1993 mega hit Mega Man X was in its earliest stages of development, it wasn't planned to star a blue bomber similar to the protagonist of the original series. The character Zero, significantly different in appearance due to his red-and-white suit and long blonde ponytail, was set to play the starring role. However, Keiji Inafune - the character's creator and primary series artist along with Hayato Kaji - feared that fans wouldn't react well to having such a different robot as the main character and intead had Kaji design a more mature-looking blue Mega Man character. Zero was placed in the game as a supporting character, and would be playable later in the series. When the original and X series were starting to lose steam in the early 2000s, Inti Creates offered him his own series where the franchise's formula could be deviated from; also of note, they later successfully returned to said formula with the downloadable Mega Man 9.
The Reploid War resulted from an imperfect recreation of Mega Man X's electronic makeup which brought about a Reploid-corrupting virus. Reploids infected became Mavericks, and were hunted by the trio of Mega Man X, Zero, and Axl, the Maverick Hunters. Eventually the leader of the malfunctioning robots, Sigma, was destroyed and the Hunters were able to eliminate the virus and go into hibernation until needed again. Mega Man Zero picks up a century later, in a world where X, the leader of the paradise-to-be Neo Arcadia, has become obsessed with destroying Mavericks and has them arrested whenever fancy strikes. Ciel, who has a connection to the X in question, finds Zero in an underground lab, where she revives him. He doesn't remember much - his name, his past, or his purpose - but he's the world's greatest hope of overcoming the government and restoring peace. The whole thing as a continuation of the Mega Man saga makes sense - the Maverick Hunters become a menace once their purpose is fulfilled. And the things that don't make sense - X's role in the conflict, for example - are explained by the end. And in the wake of chaos and disaster, Zero's and Ciel's romance is subtly hinted toward.
Zero is a joy to maneuver through levels - his movement is responsive; he can dash, an afterimage trailing behind him; he can slide on walls and jump off of them, climbing by doing this repeatedly. Zero can operate two weapons, his main weapon and his side weapon. Because of his memory loss he can only use his Buster Shot to begin with, but he quickly relearns the Z-Saber, and then gradually relearns the Triple Rod and Shield Boomerang. These weapons can be leveled up by using them to defeat enemies, and with each level gained new features are added - the ability to attack multiple times in succession or charge the weapon for a more powerful strike, namely. Thunder, Flame, and Ice Elemental Chips affect Zero's attacks against the right or wrong enemies as well. As if these abilities utilized with a little finesse weren't enough (and given the challenges presented in the missions, only a little finesse won't be), Zero collects throughout the game programs called "Cyber Elves". These help Zero in a variety of ways, be they temporary or lasting: Nurse Elves in particular lengthen or renew Zero's health bar, their exact effect varying by specific Elf; Animal Elves, then, alter Zero's abilities or provide support in battle; Hacker Elves change aspects of levels and missions, finally. Some Cyber Elves can be accessed only at certain points in the game, and some must be fed crystals dropped by enemies to evolve into their usable form. Raising and preserving Cyber Elves without using them may yield its rewards, but they're thankfully available and with limited continues one can't be blamed for using them as a crutch on a first run-through.
The resistance is the hub of the game. There, Zero can talk and receive help from those Reploids rescued from missions, save data and enter the missions themselves via the Data Room where Ciel resides, and change areas and feed or download Cyber Elves via the Trance Room. Mega Man Zero blends the level-based flow of straight platformers with the non-linear, connected aspect of Metroid. The way this is done is clever - upon completing a mission, the area in which it takes place becomes a seamless part of the accessible map. As missions are knocked out, some parts of areas become blocked off while others are opened up. The Underground Laboratory and Neo Arcadia are separate from the main map, however. Even though the missions are quite short, they require the player to be on his or her toes. In general, if a mission is particularly challenging, the boss following it is of the easier sort; if the mission seems like a breeze, the master enemy ends up making up for it - and in this way the difficulty balances itself out. The Cyber Elves also come in handy, especially the permanent ones which increase Zero's health bar, one which improves his defense, and one which places platforms where all spikes would normally be. Even with spikes out of the way and statistics altered, the game presents its challenges - bottomless pits, like spike pits, cause one-hit K.O.s, while bosses can have as much as four bars to their health meters. For the hardcore players, or those looking to unlock the Cyber Elf Jackson, these are optional temptations; certainly precious aid for a first run-through of the game.
Just like his physics, Zero's sprite moves beautifully, and the other characters are solidly animated as well. There isn't much of an attempt at texture in the backgrounds, but they're lively enough. The whole thing is colorful, especially for a game with such a dark story. The music is a big motivational factor - it follows in the "Rock"man tradition faithfully; though there aren't many tunes in the game and the ones present are often listened to in long stretches, they're fun and don't get old quickly.
There are several modes in the game - Easy Mode, which upon completion starts the player at the beginning of a game with all weapons and their effects in place; Hard Mode, another mode attainable upon completion which makes missions and bosses that much more difficult; Jackson Mode in which Zero starts with the invincibility-rendering Cyber Elf Jackson, attained by defeating the game with all Cyber Elves fully fed but unused (a hard mode in itself); and Ultimate Mode, unlocked by using every Cyber Elf in Jackson Mode and beating the game, in which a number of effects make Zero over-powerful.
The beginning of Zero's starring series is impressive. The controls make even the more frustrating parts enjoyable, and the Cyber Elf system provide a nice crutch for first-timers and the less hardcore players. As is the tradition with the series, numerous sequels would be released using the same engine and graphics while making a couple of basic gameplay modifications. Not until Mega Man ZX four years later would the experience be changed considerably, bringing the non-linear exploration back to boot.