Trix Hunter grew up in Combine-occupied territory. Raised and trained by her mercenary father, she was orphaned at age 14 when Overwatch soldiers attacked the farm where they’d been hiding. She continued training on her own, honing her skills with knives and hiring herself out as an assassin. She went against the advice of her associates by answering an ad seeking security/pest control help at Aperture Science and has been there ever since.
Kevin McAuliffe was an astronaut candidate for the Aperture Science Space Exploration Team. He was eager to follow in the footsteps of his father, but was diagnosed with acute agoraphobia and stricken from the program. In an attempt to prove the assayers wrong he booked time in the Space Simulation Room, bue to an oversight and a long weekend, he became trapped for longer than he intended. It broke his mind. He was eventually caught
killing another candidate introducing someone to the glories of space and fast-tracked to the Personality Core Transfer Program.
Lee Turret (Formerly Henry Dowd) was on the janitorial staff until getting caught in one of Medical Core’s traps. He was dragged down to Old Aperture, cut open, and had turret parts stuffed in as an attempt to make a mobile turret. The turret programming suppressed Henry’s mind/memories. Medical discarded him as a failure, but a rogue android found him and turned him into her own weapon/toy/tinkerer. He’s been Above a few times and made some friends, but his loyalty is always to his Lady.