7. Ace - Dragonfire
When the Doctor and Mel landed on Iceworld, the last thing they expected was to get caught up in an interstellar treasure hunt and scheme for revenge, yet that is precisely what happened. In the course of their investigation, they crossed paths with Ace, a teenage waitress working at an ice cream parlor. She was dissatisfied with her current life and longed for adventure - but as the Doctor quietly deduced, she was just a lost, lonely girl who missed her old life in 1980's Perivale despite her protestations. The Doctor saw great potential in Ace, so when Mel chose to leave with Sabalom Glitz, he promptly invited Ace aboard the TARDIS. Dragonfire introduced a companion who truly was fiery. Ace's affinity for explosives and adventurous spirit were well represented in her debut episode, and they were traits she continued to demonstrate all throughout her tenure as a companion. Yes, she grew and matured as a person, but the core character we met in this episode remained essentially the same.