Abel Mayford is the younger brother of Giles Mayford. Although believed to be dead by his brother, it was recently revealed that he is actually alive and now a vampire, apparently one able to age, something abnormal in his kind.
It has been implied, although not explicitly stated, that Kay is his sire, from the way the older vampire issues him commands.
He clearly has no affection of any sort towards his brother, as shown in the way he responds to being ordered to kill him. In fact Abel shows signs that he may even delight in the thought of killing his brother. Despite being a subbordinate, he also mouths off to his superiors and even goes as far as to insult them.