Serpent

Serpent (セルパン) is the head of Slither Inc. (Serpent Company in the original Japanese game, in which it is named after him). Slither Inc. works in excavation areas for the lifeline of the city. The ancient relics give energy sources to the city. Many advancements in the protection of the city were thanks to Slither Inc.'s work; even though Mavericks attacks still happen. Though Slither Inc. has acquired much power, for the people living in the frontier regions, this is not necessarily a bad thing. Serpent has amassed for himself a Mechaniloid army in his ranks that he has already used several times in the past to save countries from Mavericks even though he used the dieing ones as Cyber-elves for his plan. It is revealed that he was a former member of the Guardians. It's also subtly implied in Aile's Story and in Aeolus's Cipher that he was a Maverick survivor, and was a Mega-man because of that.

He comes into the possession of the Biometal Model W (Model V in the Japanese version), which originate from Dr. Weil (Dr. Vile) and becomes the antagonist of the game. Serpent makes it his mission to revive Model W by sacrificing Cyber-elves filled with the power of fear from those he couldn't "save" from Maverick attacks. When Vent/Aile defeat Serpent for the first time, Serpent harnesses his/her hatred and uses it as the final sacrifice to Model W, fusing with it to become a powerful robot. However, Model W's resurrection is short-lived, as he is defeated again. As Serpent dies, he declares Vent to be a Maverick for stopping progress (and mentions that anyone in humanity who has hatred in their heart "are the true mavericks"), although it is clear that this sort of progress would have destroyed the world.

It should be noted that Serpent was not necessarily evil, because it is suggested that Model W drove him insane. He thinks he's doing the right thing, because in the final battle he states he wishes to evolve the world rather than destroy it, and in Vent's story, he states that he wishes to remove all the pain and suffering. In the end, it was his own doing the undid Serpent.

Serpent makes a brief cameo in Advent after the player defeat Aeolus and gain his cipher, as one of the Chosen ones of Biometals.

Also, in Advent, it's implied that Serpent may have been revived, when Master Albert reveals that Model W's ability (as well as the true nature of A-trans), is not only to mimic all those absorbed by Model W/defeated in battle, but also reincarnate/revive said individuals.

Trivia

 * It's often a controversy as to what species Serpent is. On one hand, he has an upside down Triangle brand on his forehead, and Model W was stated to turn machines into mavericks, which would imply that he was a Reploid. On the other hand, Aile's story states that he was a Maverick Survivor, and it was stated in both storylines that Model W also turns humans into Mavericks, which also implies that he is a human, however, Model W's influence does not discriminate between man and machine as it effects being on the mental level.
 * It would be thought that Serpent was the one who created Pandora and Prometheus, but in the sequel it is revealed that Albert created them.
 * Actually, it can be assumed that Pandora and Prometheus were controlling Serpent the whole time because he seems calm, but when Pandora and Prometheus were not with him (or in close proximity, as shown in his taking Model W), he seemed crazy and didn't know what he was doing. This is also implied by some Slither Inc. employees when they said that Slither, Inc's way of operation changed as soon as Prometheus and Pandora began to work there.
 * His regular Megamerged form is almost identical to the final form of Albert.
 * His final form (as well as the method as to beat it) is also similar to Wolf Sigma. In fact, Serpent has many similarities to Sigma, both even saying "Evolution requires Sacrifice".
 * Kiyoyuki Yanada, Serpent's Seiyuu, also did the voice of Megatron in "Transformers Micron Legends" (The Japanese version of "Transformers Armada")
 * In a commercial for Mega Man ZX, Serpent, Promethus and Pandora assume a pose together that resembles Omega's first form.