Chimera

Chimera is a fictional character, He does not appear in Mass Effect 3.

Chimera, also known as Kaidan Alenko or Subject Chimera was a super soldier that was resurrected by Cerberus after his "death" on Virmire.

Apperance
Chimera is dressed in black, white and orange. This is the colors of Cerberus. He was scarred by the Virmire nuke which left him disfigured but he was changed when Cerberus rebuilt him, making him look like he was before. His armor is made of Reaper tech, impenetrable to all damage.

In battle, Chimera uses biotics and a blue lash to knock people down. He also is skilled with armed warfare.

When he was with Shepard during his short time as his old self, he wore the clothes of the Mass Effect 3 Kaidan Alenko. (he is the same person after all).

History
Chimera was once Kaidan Alenko, an Alliance soldier that accompanied Commander Shepard against the rogue spectre Saren Arterius. During the mission, Shepard went to Virmire and chose to sacrifice Kaidan instead of the other Alliance commander Ashley Williams.

There was no way that Kaidan survived...

We were proven wrong...

Becoming Chimera
In 2184 A.D., Cerberus forces found the remains of Kaidan Alenko on Virmire. He was lifeless and looked scarred and disfigured.

He was brought back similar to the Lazarus Project that brought Shepard back. He became Chimera, also known as Subject Chimera. He was implanted with advanced L2 implants, an upgrade to the implants Kaidan had.

Working for Cerberus
Chimera returned to duty in early 2186, to fight the Systems Alliance and the Terminus Systems in the name of Cerberus.

His first objective was to lead the assault on Noveria, in the Horse Head Nebula. He managed to take it over before the end of the year. Then the Illusive Man asked him to go to Grissom Academy and capture the biotic students.

Although he was unable to capture the students, he managed to capture Jack, the teacher of the students. Jack was known to Cerberus as Subject Zero.

Through Chimera's ruthless power and extreme torture, Jack became a phantom.

Citadel Coup
Chimera and the Illusive Man's other general Kai Leng, were sent to the Citadel to kill the council. However they were stopped by Shepard. Chimera met Shepard in Grissom Academy earlier but was defeated easily. With the mission a failure, Leng and Chimera returned to the Illusive Man.

Chimera used his disguise as Kaidan Alenko to get closer to the Citadel Council in order to kill them. His plan was foiled when Shepard cornered him near the Presidium and Shepard made him stand down.

For his failure, the Illusive Man ordered scientists to work on an improved, more powerful Chimera. He implanted L3 implants into him.

The Battles of Rannoch and Thessia
Chimera was chosen to stop Geth and Quarian peace by ambushing Shepard on Rannoch, this time, He managed to defeat Shepard and kill Admiral Koris, a member of the Admirality Board. (off screen)

Then Chimera was recalled to Thessia to assist Kai Leng in stealing the Prothean Beacon. Shepard lost again to the combined might of both of them.

The War for Omega
Chimera played a minor role of Aria T'Loak's war for Omega. He was the one that ordered the Adjutants against her and Shepard.

The Siege of Cronos Station and Death
Chimera was recalled to defend Cronos Station from an Alliance station. He was later joined by Kai Leng after his mission on Horizon.

He wanted to lure Shepard into a trap and kill him/her. But he never got the opportunity as Shepard was dealing with other side missions like taking over Omega.

When Shepard did finally attack Cronos Station, he/she was finally ready to kill any member of Cerberus including Kai Leng and Chimera.

He was finally killed by Shepard along with Kai Leng in the Illusive Man's room.

Trivia
The character of Chimera is inspired by this video:

How I would replace Kai Leng by The Kingdom