Jack

“And (Shepard) could, I dunno, shake her gun at them and make everyone go all happy-clappy, kum-ba-yah bullshit. Like she does.”
Jack is a powerful biotic and former member of the Normandy squad. She appears to modify her appearance to match her experiences and reflect her state of mind, particularly though tattoos and hair. In 2185, Jack sported a completely shaved head (a relic of her time in a cult) and wore what appeared to be a few belts joined together in leu of a shirt to show off her myriad number of tattoos. By the Reaper War in 2186, Jack was instead sporting an undercut, after she allowed the top of her hair to grow out, and her top covered slightly more of her skin, though still showing a large number of her scars and tattoos. Foul mouthed and sarcastic, Jack is far more kindhearted than her prickly exterior would suggest. She has an especially soft spot for children.
=== Early Life ===
Born Jennifer, last name unknown, Jack was abducted as an infant in 2165 by Cerberus Operatives off of the Eden Prime Helios Medical Facility branch when a doctor discovered her incredible biotic potential. Jack's first memories were of the facility on Pragia. There, she became the sole focus of a Cerberus program with the goal of enchanting biotic abilities in humans. She was kept isolated from the other biotic children, who were used as guinea pigs to ensure no treatments would be lethal to Jack (or "Subject Zero" as the Teltin facility called her). Many of the children died due to these experiments, or were sent against Jack in gladiatorial style matches to the death. The scientists pumped Jack full of narcotics to have her associate killing with the feeling of euphoria, and tortured her to force her to break down mental barriers to allow for more biotic power.
Eventually, the biotic children rioted and Jack escaped in the chaos, having grown far more powerful than the Cerberus operatives expected. Her rage unleashed, she killed anyone in her way before escaping in a stolen shuttle. Due to her age and trauma Jack's memories of this time would be unclear and not always correct. When her ship was discovered by another a few days later, the crew abused Jack further, before selling her into slavery.
In the time between her escape and 2185 Jack had a long and bloody criminal career, including piracy, murder, and at one point joining a cult. These experiences reinforced her belief that everyone around her was only out for themselves.
Jack never forgot the organization that kidnapped her and ruined any chance of a normal life she might have had. When Jack tracked down one of the former Teltin doctors, she came into conflict with the human-centric organization again. Though she ended up escaping being captured by Cerberus, a lucky Blue Suns mercenary was able to knock her out. 
She was brought to the Blue Sun's prison ship the Purgatory. While there, she was assaulted by a number of guards and prisoners. After she healed from their brutalization Jack murdered each individual who took part, forcing the Warden to lock her in cryostasis due to her being too valuable to space.
=== Mass Effect 2 ===
In 2185, the newly resurrected Commander Shepard journeyed to the Purgatory to pick up Jack with the intention of recruiting her for her mission against the Collectors. Warden Kuril attempted to double-cross Shepard, with the intention of selling her to either the Collectors or the Shadow Broker. Unfortunately for him, the Warden had grossly underestimated Shepard and her team's prowess in battle. Shepard fought her way to the ship's control room and released Jack from cryo. As soon as she regained consciousness, Jack blasted her way though several YMIR heavy mechs and began tearing her way though the prison.
As Jack reached the docks and discovered the Normandy waiting, the first thing she saw was the Cerberus logo painted along the side of her hull. Jack rage was apparent, but Shepard was able to convince Jack to come with them after pointing out that not only did she have the only way off, but she was willing to share whatever files Cerberus had on Jack. This almost instantly brought Jack into conflict with the Cerberus XO Miranda Lawson, who was none too pleased with Jack's attitude.
Initially Jack was extremely hostile to everyone on board the Normandy, being extremely untrustworthy. Jack was further angered and confused by Shepard's efforts to get to know her, being completely unable to determine what the Commander's angle was. As Shepard continued with her mission, helping people she came across and recruiting more squad members for their upcoming suicide mission though the Omega-4 Relay, Jack continued digging though the files. Eventually she discovered the name and precise location of the Cerberus facility that had haunted her nightmares. Jack came to Shepard with a simple demand: bring Jack to the Teltin Facility and help her plant a giant bomb there. Jack slowly revealed a level of vulnerableness and genuine distress over her memories there. Shepard agreed and made arrangements to travel to Pragia.
As they made their way though the defunct laboratory, Shepard and Jack discovered a number of left over files, many detailing the atrocities committed there. However, not all of these matched up with what Jack remembered, leaving her more and more distressed as they ventured deeper within the facility, to the point it was almost a relief to discover hostile Blood Pack mercenaries to fight. Almost. In the deepest section of the building, they discovered a fellow survivor of the Teltin project, a young and broken man named Aresh who had hired the mercenaries as part of a plan to restart the experiments there in a desperate and deranged attempt to derive meaning from his suffering. After dealing with Aresh, Jack took a few moments to grieve over her experiences there before blowing the facility up.
Shortly afterwards, Jack and Miranda nearly came to blows on board the Normandy due to Miranda's continuing loyalty to Cerberus and Jack's own experiences with the organization. Shepard was able to defuse the situation peacefully without taking sides. 
After surviving the assault on the Collector base, Jack was contacted by the Systems Alliance, and was offered a position at the Jon Grissom Academy. Jack accepted, presumably due to the data at the Teltin Facility pointing towards some of the people there trying to piggyback off of Grissom's Ascension Program. While she was there, her biotic students blossomed under her "unorthodox teaching style" and Jack herself felt a fierce protectiveness over "her kids".
=== Mass Effect 3 ===
After the Reaper attack on Palaven, Samantha Traynor discovered an altered turian signal responding to an evacuation request from Grissom Academy. With EDI's help, she came to the conclusion that the signal actually came from Cerberus, leaving the students at the Academy in danger. The Normandy responded and discovered the facility under attack by Cerberus troops. Shepard ended up fighting her way to a group of students putting up a resistance against their kidnappers and discovered Jack leading her students in their defense. Together, Shepard and her squad assisted Jack and her students in their escape from the academy.
While the after the encounter with Shepard's clone, Jack invited Shepard out to the Armax Arsenal Arena to blow off steam together. When Shepard held a party at the apartment she had been gifted by Admiral Anderson, Jack was among the guests. It seems likely that this is when she and Miranda might have decided to become romantically entangled, but this is not been confirmed.
== Every One That Asketh ==