Gil and Gui were twins who illustrated the dangers of the sort of extensive in-breeding which the revenant family indulges in. Both mentally retarded and disfigured, the children were turned over to their Uncle Dagobert with the hopes that he might make something out of the boys. This was rather literally what he did, as he was the tender of the family's kennel at that point and a master at the art of vicissitude. Through judicious use of blood and experimentation he molded the young boys into forms resembling the hell hounds he'd bred and altered, and presented the finished product to Marie-Thérèse, who was so taken by the loyal little pups that she allowed them to be Embraced, to serve as her personal pets.
Raised as animals and given animal form, the boys are barely human in behavior. Rather, they are more like what they resemble, large and vicious hell hounds, waiting at the beck and call of the matriarch and ready to defend the family plantation and its members to the death. Time means little to them and they have very short memories. The battles they've fought and the death they've seen linger only as emotional connections to particular voices, smells, and sounds.
Gil is the more friendly of the twins, prone to approaching strangers and offering his discomforting smile (consisting of three rows of shark-like teeth). He is also the one more prone to wander the grounds and explore the house, which has gotten him into a bit of trouble in the past. His mistress would likely stop him from doing so if it didn't serve the dual purpose of allowing him to stumble in on interesting scenes every now and then and instilling in the family a sense of paranoia concerning privacy.
Gui is more sedate, more vicious, and stays very close to his mistress, guarding her side. He won't, without her specific command (and even then, very reluctantly) leave her side. He also tends to raise his hackles at anyone who raises her voice to her or does anything remotely threatening, and has even attacked those whom he senses mean his lady harm, whether they are being hostile or not. Of the two, he is the one most feared by the family and its associates. This is somewhat ironic, given that he is a very direct and brutal fighter, who is likely to kill his opponent outright, while his more whimsical brother is more prone to play with his catches and torture them a bit out of curiosity.
The boys are, of course, unwavering loyal to their mistress, and show the proper respect to the other Tzimisce in the family. They also like Father Gossett and Uncle Henri, though they don't take orders from either of them. Always, though, the orders of Marie-Thérèse take precedence over all others and they would likely eat Emma alive if she so commanded, a fact which isn't lost on the other members of the family.
They are both very fond of Rodolphe and are as loyal to him as they are to Jean-Michel, though they do not particularly like Ida Prejean, whom they feel is a threat to their mistress. They behave towards Émeline as they would towards their domitor, but have a sense for knowing when she speaks for herself and when she speaks for the matriarch and only respond in the latter case (though the former grows much less frequent).
They are cool towards Gerard, mostly because he isn't fond of them, and are outright hostile towards Raoul for reasons even their mistress isn't entirely sure of. They are also fond of Jocelyn and her sisters and would defend them and their children to the death. Lastly, they are cool towards Sevis but rather like Beranger, which isn't an uncommon attitude in the household. Towards other members of the family, they are generally very suspicious if they aren't part of the Lafayette brood and some of the other branchs, particularly the Galveston one, they are outright hostile towards (likely picking up on the emotions of the other Tzimisce).