Who is Boo from Brother Odd and what is their importance?

Boo is an entirely-white Shepherd cross, Boo belongs to and lives at the Abbey. He is one hundred and ten pounds and as graceful as he is brave. With clear blue eyes, he and Odd have bonded to a degree that he even hides in Odd's suite at times and sleeps there when he feels the need for companionship. Boo is the first and only ghost dog Odd has ever seen.