Sigma

Sigma is an evil robot that, like X, has a free will. Unfortunately, he uses that free will to wage a war against the humans. He wishes to destroy all humans to make reploids the supreme masters. Sigma was once a Maverick Hunter, but he got infected by a virus Zero had with him in the battle when he found Zero damaged and out of control in some ruins. Later, Sigma created the Sigma Virus. He seens to know that a lot about Zero, like that he is Dr. Wily's last creation. Sigma and Zero were destroyed in the end of Mega Man X5, but Gate ressurected Sigma in Mega Man X6. Sigma is destroyed for good in Mega Man X8, as Lumine is revealed to be the first of the "turn-Maverick-at-will" Reploids.



Sigma's Weaknesses
Because of his multiple incarnations in the games, his weaknesses in each game will be listed here.

Mega Man X1: Mega Man X2: Mega Man Xtreme: Mega Man X3: Mega Man Xtreme 2: Mega Man X4: Mega Man X5: Mega Man X6: Mega Man X7: Mega Man X8:
 * Form 1: Electric Spark
 * Form 2 (Velguarder Sigma): Rolling Shield
 * Form 1 (Neo Sigma): Sonic Slicer
 * Form 2 (Sigma Virus): Strike Chain
 * Form 1: Electric Spark
 * Form 2: X-Buster
 * Form 1: Spinning Blade
 * Form 2 (Kaiser Sigma): Only charged X-Buster can harm him, unless you have the Z-Saber...
 * Form 1: Drill Crush/Dash/Marine Tornado
 * Form 2: X-Buster/Z-Saber/Marine Tornado
 * Form 1 (Grim Reaper form): Rising Fire (X)/Ryuenjin (Zero)
 * Form 2 (regular): Lightning Web (X)/Kuuenbuu (Zero)
 * Form 3: X-Buster or Nova Strike (X)/Z-Saber, Shippuuga, and Kuuenbuu (Zero)
 * Form 1: Tri-Thunder (X)/E-Blade (Zero)
 * Form 2: X-Buster, Nova Strike, or Spike Ball (X)/Twin Dream (Zero)
 * Form 1: Metal Anchor (X)/Rakukojin (Zero)
 * Form 2 (giant metal skeleton): Ground Dash (X)/Ensuizan (Zero)
 * Form 1: X-Buster (X)/Hieijin (Zero)/A-Bullet (Axl)
 * Form 2: X-Buster (X)/Z-Saber (Zero)/G-Launcher (Axl)
 * Sigma Copy (in Gateway): Shadow Runner (X)/Rasetsusen (Zero)/Black Arrow (Axl)
 * Real Sigma (in Sigma Palace): Shining Ray (X)/Tenshouha (Zero)/Ray Gun (Axl)

Trivia

 * Sigma (&Sigma;) is the 18th letter of the Greek alphabet. The &Sigma; symbol is often used on Mavericks, most notably on Vile.
 * Sigma is called Cigma in the Mega Man cartoon series.
 * According to Keiji Inafune, Mega Man's creator, Sigma's appearance was largely inspired by Doc Robot from Mega Man 3, one of the few antagonists in the series not designed by Inafune himself. He then stated that although Mega Man 3 is his personal favorite of the original series titles, Sigma is the only creation of his or any of his staff to borrow from that game's wealth of characters.