To circumvent The Doctor's obliteration by photon-hating humanoids, Seven secretly becomes his bodily host. In turn, The Doctor's personality dominates.
When attacked by a species that detests holographic beings, Seven download the Doctor's program into her implants. Kim and Seven are captured and Seven begins to exhibit some of the Doctor's personality traits. On Voyager, Tuvok finally starts to exhibit Pon Farr, the Vulcan mating time.—laird-3