Sully is a dog. More specifically, he is a bloodhound. He is supposed to be the best tracking dog around.
In Chapter 25, Jeff Bussey has decided he has to run away from the rebel army because he has been found out. If they catch him, he is going to be executed for sure.
The rebels are having a hard time finding him so they send for Sully to track Jeff. Sully eventually finds Jeff because he is such a good tracker. Luckily, Jeff has a magic touch with animals and Sully loves him instead of barking and telling the trackers where he is.
So Sully becomes Jeff's dog.