Redheads are Fixed Points in Time (Doctor Who)
First appeared when the Tardis beamed her aboard due to being of the same kind of particles that the Tardis emits. This was due to some kind of invasion by an alien race.
At her last appearance, Donna was left in the Tardis when the Daleks invaded the earth after rebuffing the Cybermen's offer of alliance. The Tenth Doctor's hand then was activated by his regeneration energy to form the human version of the Doctor using Donna's DNA as the template.
First appeared when the Tenth Doctor regenerated into the Eleventh. Because the Eleventh Doctor complained about not being ginger, the Tardis aimed for Amelia Pond's house because of the cracks on her bedroom wall.
Last appearance was when both she and Rory Williams, her husband, were transported to late 1940s Manhattan by the cherub form of the weeping angels. Due to the time anomaly, the Doctor cannot fly the Tardis back to that point in time. Only Melody Pond's time vortex manipulator can gain her access to the time period.