Dougal and Colum Mackenzie are the brothers in charge of Castle Leoch, the stronghold of Clan Mackenzie. Colum is the chief, and Dougal is his right-hand man. Well, he's more like Colum's everything since he acts as landlord, brute force, and even has Colum's children for him (Colum being unable to have an heir of his own). As Claire realizes, "You swore to be his arms and legs […] why shouldn't you be his cock as well?" (34.127). She kind of has a point.
At first, Claire believes that Dougal is working against Colum, secretly raising money for the Jacobite uprising, but she learns that Colum is in on it the whole time. Still, that doesn't stop Dougal from often acting with his own interests in mind, especially when it comes to Claire. He saves Claire from pursuers on the night of the clan Gathering… but then he kisses her. He helps her again later, but decides to leave Jamie for dead so that he can marry Claire himself.
Basically, he's not to be trusted. Jamie even suspects that Dougal tried to kill him with an axe one night, to ensure that he would be laird of Leoch instead of Jamie. He says he didn't do it, but we're not sure we believe him.
Colum Mackenzie has Toulouse-Lautrec syndrome, which matters insofar as it renders him the brains of the outfit, while Dougal is the brawn. His son Hamish is not his own, which Claire spots right away, thinking he looks like "a smaller version of his uncle Dougal" (6.6). And that, of course, is exactly what he is.
Perhaps because of his physical reality, Colum is more levelheaded than Dougal, and therefore chosen to be in charge of the clan. Who knows what ill-advised battles Dougal would lead them into were he chieftain. Colum is used to the power he has, and Claire observes that he is, "Law, jury, and judge to the people in his domain—and clearly accustomed to having things his own way" (7.13). You judge him by his size?
Once Claire leaves Leoch, however, we don't see Colum again. His short legs keep him in the castle, away from the action of the book.