Staff Lieutenant Kaidan Alenko is a crew member on the SSV Normandy at the start of Mass Effect. He was exposed to element zero in utero after his mother was near a crashed transport incident in Singapore. He attended the Biotic Acclimation and Temperance Training (BAaT), where he learned to control his biotic ability and studied advanced technology. His experiences at BAaT led him to become more cautious and conservative, believing that humans should take a slow entrance into the galactic state of affairs. After leaving BAaT, Kaidan was assigned to the Normandy under Captain Anderson and served alongside Commander Shepard. Together, they discover a geth invasion of Eden Prime, thus starting the events of the Mass Effect.
Kaidan has the Sentinel class, meaning he has skills related to technology and biotics. He has no weapons or armor skills, and can only wear light armor. A female Commander Shepard can have a romantic relationship with Kaidan; in fact, it becomes clear early on that Kaidan is attracted to his commanding officer. A female Shepard can even persuade Kaidan to take a slightly more pro-human viewpoint in his politics.
Kaidan is one of the two characters that you can leave to die in the explosion on Virmire; the player must decide to save either Kaidan or Gunnery Chief Ashley Williams.
He's voiced by Raphael Sbarge.
Alliance Loyalty (Mass Effect 2)
After Commander Shepard was declared killed in action, Kaidan gradually moved on by remaining in the Systems Alliance and he was eventually promoted to Staff Commander. There were rumors of Shepard's miraculous return and that he was potentially working for Cerberus now - the pro-human terrorist organization and the main suspect in the disappearance of habitants of several human colonies. There were also rumors that the next attack would occur on the colony Horizon so he was dispatched to investigate. Despite witnessing the true assailants first hand, Kaidan still couldn't accept that Shepard worked under a Cerberus banner and swore his allegiance to the Systems Alliance before parting ways with him.
If Shepard and Kaidan were romantically involved in the first Mass Effect, Kaidan will not only embrace her before ventilating his anger, he will also write her a letter apologizing for the way he reacted and emphasizing how much she means to him. Shepard will also keep a framed photo of him in her quarters so long she remains loyal to him. If the player romances another member of the crew (not including Kelly Chambers), the picture frame will be flipped face down on the desk. If the player remains loyal, on the other hand, Shepard will spend her last moment before the suicide mission looking at the picture.
Mass Effect 3
Little is known so far, aside from that Kaidan will return as a squad member if he survived Virmire. He becomes a SPECTRE during the game. He is available as a romantic partner for Shepard regardless of gender.