Signas

Signas, formerly known as Sigma Enigma, is the commander of the Maverick Hunters, and one can consider him both Dr. Cain's greatest creation and biggest failure.

Pre-Rise of Sigma
As the leader of the Maverick Hunters, Sigma was originally the most advanced Reploid of his time. Dr. Cain implanted the latest circuitry designs into Sigma, which were theoretically supposed to prevent him from going Maverick. He was placed in charge of the Maverick Hunters, and led them to victory multiple times.

One day, the Maverick Hunters received reports of an overwhelmingly powerful "Red Maverick" that had destroyed an entire squadron of Hunters (the Garma unit). While investigating, Sigma, along with his unit, tracked it down to an abandoned laboratory. Sigma then left to confront it alone and personally deal with it, in order to prevent other Reploids' deaths at the hands of the Maverick. It is here that Sigma encountered Zero, the last product of Dr. Wily. Zero, being the original carrier of a deadly virus, was uncontrollable, unleashing wanton destruction wherever he went. But now Sigma stood in the path of Zero's rampage.

Sigma was beaten badly by the maddened Zero and sustaining heavy damage. A 'W' appeared in the gem on Zero's helmet causing Zero pain, possibly due to the virus he was infected by, and Sigma took the opportunity to smash this crystal: a destructively devastating blow. Most likely by this punch, the Maverick Virus was transferred directly to Sigma, infecting him. Zero lost consciousness after taking Sigma's punch, and Sigma limped away from the battle, ordering his men to take the body of Zero back for study as well as to contact Dr. Cain notifying him of this, also angrily dismissing them when his men expressed concern for his injuries; Zero recovered after repairs, but suffered from amnesia as he could no longer consciously remember his original purpose or any earlier memories. The virus began to mutate and adapt to Sigma's own programming, eventually taking his persona to form the Sigma Virus, which allowed him to manipulate and control Reploids, bending them to his will. After this, Sigma began to feel that humanity was keeping Reploid potential down, setting the stage for his eventual war against humanity.

Rise of Sigma
Sigma's advanced circuitry seemed to do its job, as he remained stable for months after the lab incident. However, he gradually succumbed to the virus and grew more volatile. Without warning, he leads a mass revolt among the Reploids. Many Maverick Hunters follow Sigma due to them being corrupted by the virus, leaving Zero, who by this time has reformed and become a powerful Maverick Hunter, in charge of the organization. Mega Man X agrees to join Zero in the fight, and X and Zero battle through Sigma's Maverick regime together. Unbeknownst to Zero, however, X salvaged the personality chips of the Mavericks in a final hope to save them from themselves, which he succeeds, but the Mavericks, out of shame, decide to return undercover as the X-Hunters. It is X who finally confronts Sigma alone in the Sigma Palace.

When met with X, Sigma first sends his pet, Velguarder, to test his power. Once it is destroyed, Sigma personally confronts him wielding his beam saber. After his first body is destroyed, Sigma's leftover head attaches itself to a giant canine body called "Wolf Sigma". X defeats this form of Sigma as well, salvaged Sigma's personality chip and escaped with it while Sigma's physical body and his base were blowing to smithereens. It is then when X learns that Zero was the original carrier of the virus, and it is then when it's revealed to the X-Hunters that the virus caused Sigma's mind to split apart: X salvaged Sigma's reasonable side, and Sigma's feral side became one with the virus.

Lazarus Zero
Sigma joined the X-Hunters shortly after his defeat, under the alias of Signas; despite some proposals from the X-Hunters, Signas decided to start as a green recruit all over again, mostly out of shame for himself. Signas, however, impressed all Maverick Hunters with his sword and command skills.

After having her heart and an arm torn apart by Flame Stag (her former mentor), Alia learns Signas' true identity via X. She joins the X-Hunters afterwards.

Doppler Effect
The events of Lazarus Zero ended up with Signas receiving a massive promotion, becoming vice-commander of the Maverick Hunters. After X was demoted to Navigator duty, Signas regained the position of commander he had prior to going Maverick as Sigma.

Rise of Repliforce
Dr. Cain also learns about the X-Hunters. Signas is the second one to tell X that Dr. Cain earned his part of truth at that point.

Personality
Sigma's original personality (which becomes Sigma's reasonable side during Rise of Sigma and, as such, was passed onto Signas) was fitting to Sigma's original purpose as commander of the Maverick Hunters. He has a calm, calculated demeanor, handling all Maverick problems professionally. He is almost never seen to lose his cool, the closest to doing it being when he smashed the gem in Zero's helmet. Nonetheless, the virus may have already started to corrupt Sigma at the time.

Another trait originally belonging to Sigma and passed onto Signas after Rise of Sigma was caring for his subordinates. He seems to trust and know them very well. It shouldn't be a surprise, however, since several units are made of Hunters that once went Maverick, but then reformed and became X-Hunters, so Sigma knew them from before going Maverick. This is apparently seen during his first encounter with Zero: after Zero decimated Garma's unit, Sigma stepped in, since he couldn't afford losing any more Hunters.

Trivia

 * Signas being Sigma's reasonable side is one of the weirder things at X Chronicles. According to Cyrex, Signas was too interesting of a character for the small role he has in the games, and that he believed Sigma's authority as the leader of the Maverick Hunters "would give Signas an edge when it came to wrangling X if needed".
 * This makes Signas one of the characters adapted in a unique way to connect him to a Maverick fans would never expect for that connection to exist, other characters being Alia (she was trained by Flame Stag), Slash Beast (he's a clone of Neon Tiger), Colonel and Iris (they were created by Gate).
 * According to X, Sigma's last name is Enigma.