Lena Oxton (call sign: "Tracer") was the youngest person ever inducted into Overwatch's experimental flight program. Known for her fearless piloting skills, she was handpicked to test the prototype of a teleporting fighter, the Slipstream. But during its first flight, the aircraft's teleportation matrix malfunctioned, and it disappeared. Lena was presumed dead.

She reappeared months later, but her ordeal had greatly changed her: her molecules had been desynchronized from the flow of time. Suffering from "chronal disassociation," she was a living ghost, disappearing for hours and days at a time. Even for the brief moments she was present, she was unable to maintain physical form. Sometimes she had strange dreams about the past, including being a kitchen maid at a large house in the country.

Ref: https://www.youtube.com/watch?v=23HdsSDZMws