3. Leela
Leela was a far cry from the average Doctor Who companion. This fearless warrior of the Sevateem considered it her duty to protect the Doctor from the dangers they often faced quite a role reversal for the Doctor, who was used to reassuring companions that he would look after them! The Doctor appreciated her company, though, and took it upon himself to educate Leela in the ways of her more civilized ancestors. This primarily consisted of teaching Leela to view things in light of science rather than magic, but he also worked to make her less violent. Leela would attack anyone she thought was an enemy on sight, prepared to kill without a second thought. Her violent tendencies clashed with the Doctor's more peaceful nature, but after a while she decided that maybe the Doctor's way wasn't so bad after all. After helping the Doctor stop the Sontaran invasion of Gallifrey, Leela decided to stay behind and marry Andred, a Chancellery guard she met during her time on the Doctor's home planet. Such an announcement was naturally surprising coming from someone as independent as Leela, and we can only wonder how she adapted to the stricter civilization of the Time Lords after being raised as a warrior and hunter. Another question to consider is whether or not she played any role in the Time War. If she was still alive during the battle, it's easy to imagine her right on the front lines.