The Tucker Parents
The Tucker family moves to Florida because the scientist father, Dr. Tucker, is convinced that deer are able to live in Florida. He moves the family from Vermont to Florida, where he raises deer in a pen and eventually plans to release them into the wild. His wife, Mrs. Tucker, is also a scientist and helps him with his research. The Tucker parents are supporting characters who are just as confused by the mysterious nightly events around their home as their children. It is their children who ultimately get to the bottom of the mystery.
Grady and Emily Tucker
Grady Tucker is 12 years old, and serves as the hero of the book. His sister, Emily, is 16 and is only a supporting character. Grady is at the center of the action, investigating the strange animal sightings and killings around the swamp. He's the one who sees the shadowy figure bring the mangled animals to the deer pen each night. Grady's investigations lead him to discover the werewolf in the swamp. Grady is attacked by the werewolf, and at the end of the book, the plot reveals that he has been turned into a werewolf.
Will and Cassie
Will and Cassie are both neighbors to the Tucker family, though they are not related. In fact, they don't even like each other. Will tells Grady that Cassie is a "weird" girl. Cassie is the one to first suggest that the animal killings are caused by a werewolf. By the end of the book, Grady discovers that Will is the werewolf after watching him transition in the swamp. Will is the werewolf who attacks Grady.
Wolf and the Swamp Hermit
Two other mysterious figures appear in the book: Wolf, a 100-pound dog who shows up one night and is adopted by the Tucker family, and the Swamp Hermit, a man who lives in the swamp and is believed to be crazy. The Swamp Hermit chases Grady and Emily when he sees them in the swamp near the beginning of the book, and the children suspect him of being the werewolf. When Grady fights with Will, who is in werewolf form, the dog jumps in to fight Will and protect Grady. He ends up pushing Will into the swamp, where he drowns. After Grady becomes a werewolf, he enjoys running through the swamp at night with Wolf, the dog, as his companion.