Mrs. O'Leary

Mrs. O'Leary is the pet Hellhound of Percy Jackson. In the Camp Half-Blood universe, hellhounds are usually enemy creatures, and almost always against the protagonists. Mrs. O'Leary who happens to be a even larger Hellhound happens to be extremely nice. She was at first a pet of Quintus. But after his death, he was passed onto Percy. Whenever Percy was away from Camp Half-Blood she was in the care of Charles Beckendorf. In the second series, she comes back, always as helpful. Percy can call her whenver he pleases.