Kaidan Alenko

Kaidan Shepard (né Alenko)  is a human Sentinel and a Systems Alliance Marine. While serving aboard the SSV Normandy, he is a Staff Lieutenant and head of the ship's Marine detail. He's also an initial member of Commander Shepard's squad. Kaidan is a biotic wired with the controversial L2 implants, which are known to cause severe neurological damage to the user. However, he is fortunate enough to only suffer from occasional severe migraines. He is Commander Shepard's main love interest.

History
Kaidan was born in 2151 into a family already familiar with space: his father served in the Alliance military. After his mother was downwind of a transport crash in Singapore, Kaidan was exposed in utero to element zero and beat the odds, gaining biotic potential instead of terminal brain cancer. At the time little was known about human biotics, but when the possibilities became better understood, a company called Conatix Industries was set up to track accidental eezo exposures. Children who showed enough potential to be worth augmenting with implants were 'encouraged' to attend BAaT (Biotic Acclimation and Temperance Training) which was conducted at Gagarin Station and nicknamed 'Brain Camp' by its attendees. As Kaidan tells Shepard, "bunch of guys in suits show up at your door after school. Next thing you know, you're out on Jump Zero."

Along with several other teenagers, including a beautiful, gentle girl named Rahna, Kaidan began his training but things went bad almost from the start. Conatix wanted to speed the work along and brought in an ex-military turian, Commander Vyrnnus, who was an expert in biotics but also a veteran of the First Contact War who disliked humans. Vyrnnus introduced himself by saying "I was at the helm of the dreadnought that killed your father", and Kaidan retorted that his father hadn't even been in the war. Vyrnnus had it in for Kaidan after that, and began pushing the teenagers very hard. Kaidan recalls: "You either came out a superman or a wreck. A lot of kids snapped. A few died."

Finally, the inevitable happened and Vyrnnus went too far. Rahna, who had become close to Kaidan during their training, reached for a glass of water instead of moving it biotically. Vyrnnus punished Rahna by breaking her arm and Kaidan tried to defend her, only to receive a beating from Vyrnnus. When the turian pulled a knife, Kaidan lost control and hit him with a full biotic kick, breaking Vyrnnus' neck. After Vyrnnus' death, BAaT was shut down and the records were sealed to hide the Alliance's mistakes. Though Rahna was safe from Vyrnnus, she was afraid of Kaidan after that and the two parted ways.

Kaidan dropped off the radar for a while to figure things out, feeling lost and paralyzed after Jump Zero, but eventually came back to the Alliance in 2173 — on his terms this time — because he wanted to serve. Refusing further biotic training or an implant retrofit, he enlisted in the Alliance Navy and worked his way up to Lieutenant, gaining several special commendations in the process despite the health problems he suffered with his L2 implants; these manifested as intense migraines that left him sensitive to bright lights and noise. Finally Kaidan was transferred to the SSV Normandy under Captain Anderson, where he later worked alongside Commander Shepard.