Red Dwarfs were red-bearded Narnian Dwarfs.
Red Dwarfs were generally cheerful, stout-hearted, and hopeful. Their anger was easily roused but short-lived; they were never sullen or vindictive. The best recorded example of this temperament is that of Trumpkin. There is no recorded instance of an evil Red Dwarf, although it is definitely possible that such instances occurred.
For more on the temperaments, skills, and history of Narnian Dwarfs in general, see Dwarf.
In the 2005 film, The Lion, the Witch and the Wardrobe, Red Dwarves were first seen in Aslan's camp, where they were building their weapons, when the Pevensies and the Beavers arrive to meet Aslan for the first time. Red Dwarves later took place in the First Battle of Beruna. They were then seen at the coronation, where the Pevensies became the Kings and Queens of Narnia.
The temperament of Red Dwarves is very like that caused by the Sanguine Complexion, one of the four archetypal medieval temperaments described in Chapter Seven of C. S. Lewis's The Discarded Image.