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 robots the supreme masters. Sigma once was a former Maverick Hunter, but he got infected by the 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.

Fun facts
Sigma is part of the greek alphabet.