Amun and Kebi are both prominent members of the Egyptian coven. They are survivors of the coven that was destroyed by the Volturi when they warred against the Romanians and the Egyptians. Other than the Volturi's ancients and the Romanians, Kebi and Amun are two of the most ancient vampires in history.
Amun and Kebi were both ancient Egyptians. Amun was one of the first who established the Egyptian coven that warred against the Romanian coven. Kebi was an Egyptian slave chosen by Amun to make her his mate; he chose her because of her beauty and loyalty, which was subsequently magnified by her transformation. However, their relationship was never one of equal.
When the Volturi established the laws and conquered the Romanians, Amun knew that his coven would be their next target and quickly swayed to obey their new laws, and Kebi followed him without question. The rest of their coven was massacred after that.
With only Kebi and Amun left, they re-established what was left of the coven by trying to find talented members. Eventually, they came across Demetri, but he was taken in by the Volturi sometime later. Amun gave up hope of ever finding special talents -- until he came across Benjamin, a boy with more special power than anyone could have imagined. Kebi did not enjoy Benjamin's involvement because he was constantly receiving Amun's attention, but she kept her feelings to herself. Five years later, Tia joined their coven as Benjamin's mate. Although Amun wasn't fancy of the idea, he realized the benefits of having Tia in their coven; subsequently, Kebi became content that Benjamin's bond to Tia weakened his bond to Amun, so she could have him more to herself.
When the Cullens sought the Egyptians out for aid under the Volturi's threat, Amun is immediately resistant to the idea even after the proof was presented, but Benjamin's subtle threat of disbanding their coven forces him to stay, and Kebi stays with him. They mostly stay to themselves and rarely interact with the other witnesses.
During the confrontation, Amun gives his testimony to Aro, then leaves with Kebi, leaving behind Benjamin and Tia. After the situation resolved peacefully, Benjamin and Tia run after them. In the movie adaptation, Amun and Kebi remain with the Cullens throughout the entire confrontation, but leave right afterwards.