Sigma, an Irregular Hunter with the best ability, has performance that outperforms any Repliroid. It also tends to focus on developing large bodies to develop a variety of weapons, especially to defeat X. Some Repliploids cooperate with Sigma and using them he aims for destruction of humanity.
シグマのイレギュラー化 (Sigma's change into an Irregular/Maverick)
Dr. Doppler found out that the identity of Sigma is a malignant virus, this virus has the property of infecting any Mechanoloid and Repliroid and becoming irregular. Later, subspecies such as Nightmare virus, Zero virus, etc. occurred. It is said that Dr. Doppler completed the antibody just before his death, but it seems that it was not perfect.
Sigma, who was indifferent to X at the time of the first rebellion, recognized X as a strong Repliroid in repeated battles. The obsession to try to defeat X is very strong, helping with the height of pride. Meanwhile, against Zero there is an intention to awaken the nature of Zero conflicting with X, and he continues to invite investigations such as repeatedly conducting surveys on Zero's past and encouraging awakening.
Sigma was said to have the most enhanced mind brain
Since Sigma himself is not damaged, it was also used as a weapon
With DNA information from the Sigma Virus and Zero Virus, Gate created the Nightmare Virus