Sigma

Sigma (or Shiguma in Japanese) is the main antagonist of the Mega Man X series. He acts as a replacement for Dr. Wily in the first Mega Man X game.

Sigma (Mega Man X)
Sigma sends Velguarder to test X's worthiness, before confronting him with a beam saber.

After defeating Sigma, he turns into Wolf Sigma and is similar to the battle with Gamma.

Neo Sigma
Sigma said that he'll watch Zero die once again if Zero does not follow him.

Neo Sigma will have claws to attack Zero, like Slash Man in Mega Man 7. Zero must keep attacking until he fights the green Sigma Virus.

Other media
Sigma's name was misspelled as Cigma in Ruby Spears' Mega Man and only appeared in Mega X.

Sigma also appeared in the twenty five minute long movie, The Day of Σ.

Sigma only appeared in the Rockman X manga.