Who are Havey and Marker in the novel, Whittington?

Havey and Marker are two dogs that, at least in their initial appearances, are both angry and violent. Havey is a sometime companion of Bernie's, who makes her home primarily at the gas station where he works but occasionally accompanies him to the barn to visit the other animals. Her frequent violent attacks on The Lady, emerging from frustration and loneliness, are halted by the intervention of Whittington, acting out of respect for his new friend (The Lady). Eventually, Havey becomes a welcome and indispensable member of the barn family, taking care of Theo and helping Coraggio back to life and health. Her story manifests the book's thematic concern with friendship by illustrating how a negative attitude can be transformed by its active presence (i.e. Whittington acting out of friendship for The Lady) and even how physical suffering (i.e. Coraggio's wounds) can be healed by compassion. Marker, unlike Havey, is just plain mean, and one day attacks the animals in the barn for no reason whatsoever. It's Marker who causes the life-threatening and voice-destroying injuries to Coraggio. He heightens the value and depth of Havey's transformation through contrast; in other words, he shows how significant her movement towards compassion is by acting entirely WITHOUT compassion.