“Always try to nice but never fail to be kind” is a key part of the Doctor’s ethos, so it’s imperative that their friends share that sentiment. The Doctor’s companions are exposed to the best – and worst – of the universe, meeting legends of time and space as well as some of its most treacherous and morally bankrupt monsters. It’s essential that any companion of the Doctor treat the people they meet on their journey with the respect and dignity that they deserve, regardless of how different they may be.
When the Doctor and Donna arrived on the Ood Sphere, Donna was disgusted by the future humans’ treatment of the Ood as "servants of humanity". On his previous encounter with the Ood, the Tenth Doctor never questioned it, which surprised Donna. This gave him the call to action he needed to help them. Despite her initial reticence around these odd aliens, Donna showed them immense compassion when she discovered they had been lobotomised by Ood Operations and their hind-brains replaced with their characteristic translator balls. Together, she and the Doctor freed the Ood, enabling their song to resonate throughout the galaxies.
Rose Tyler was also deeply empathetic, going so far as to display compassion towards one of the Doctor’s deadliest enemies, a Dalek. And luckily, it was that compassion that even saved her from it. With scars still fresh from the Time War, the Ninth Doctor was overwrought with vengeance when he discovered that just one mutant pepper pot had survived damnation. However, this Dalek had become something new. Having restored itself from Rose’s DNA, the Dalek was quickly evolving into something a lot more… human. It spared Rose’s life and she returned the favour by talking the Doctor down from killing it. She showed a broken Doctor that even a Dalek is capable of change and so her compassion saved him too.
A strong moral compass