The Paternoster Gang
The Paternoster Gang is a trio of characters that were originally summoned by the Eleventh Doctor for the Battle of Demon's Run. They include Madame Vastra, Jenny Flint, and Strax. Vastra and Jenny are a cross-species (Silurian and human) married couple, while Strax is a Sontaran who was forced to become a nurse. They were all called on because they owed a debt to the Doctor, and they helped to rescue Amy and her baby from Madame Kovarian.
After Strax seemed to die in the battle, he ended up moving in with Vastra and Jenny as their butler. Vastra served as a detective in London, identifying and then eating the worst criminals (including Jack the Ripper), and all three continued to appear throughout Steven Moffat's time as showrunner as the Doctor's friends and support system.
Altogether, the trio were only in five episodes, from Series 6 through 8, ceasing to appear after helping Clara come to terms with the Doctor's newest form. This was a shame, since it seemed like they were going to continue through Peter Capaldi's time as the Doctor as well. All three are useful in a fight, between Vastra and Jenny's fighting skills and Strax's medical capabilities, but they are also usually introduced when somebody needs to learn a lesson about how different people (and species) can learn to co-exist.