Who is Faro from Z for Zachariah and what is their importance?

Faro is a dog that was once owned by Ann's cousin David. Faro disappeared shortly after David and the rest of the family went in search of survivors. Ann thought he had gone after David. However, Faro returns the same day Mr. Loomis comes into the valley. At first Ann is happy about this. However, when Ann is forced to run away from Mr. Loomis, she realizes that Faro will be a weapon against her as Mr. Loomis trains him to track for him. In the end, Ann is forced to kill Faro by causing him to fall into the radiated Burden Creek.