Elizabeth Shaw

Liz Shaw is a highly intelligent Cambridge scientist who was recruited by the Brigadier, and she became UNIT's scientific expert. The arrival of the third Doctor made her his assistant for a while. Having helped the Doctor defeat several alien threats, she left UNIT to pursue her own career at Cambridge.

Liz Shaw was in Spearhead From Space to Inferno, plus The Five Doctors (as a "phantom"). Excluding Inferno: Episode 5 (where she only plays Section Leader).