The Walkers are three powerful Outsiders. They are He Who Walks Beside, He Who Walks Behind, and He Who Walks Before, the latter of which Harry Dresden calls "Sharkface". They first appear in Storm Front.
Very little is known about Walkers. However, Dresden faced one, barely surviving, when he was sixteen. Three of them are known in the series: He Who Walks Behind, He Who Walks Before, and He Who Walks Beside.
They are capable of powerful psychic attacks and enjoy launching them. They can leave a mark on those they hunt that can be seen with the Sight, or through the use of the ThreeEye drug. According to Lasciel, He Who Walks Behind is an Outsider, "the most potent of Walkers, and a powerful knight among the ruling entities"—and Dresden overthrew him.
In the series
In Blood Rites, using an entropy curse in the Deeps of the Raith estate, Lord Raith has Madge Shelly call up He Who Walks Behind to destroy both Thomas Raith and Harry Dresden. He Who Walks Behind escaped instead and warned Dresden that he will return some day.
In Cold Days Harry Dresden faces off Sharkface several times in the book. He first faces him in McAnally's Pub. After He Who Walks Before leaves the bar, McAnally lets Dresden know there are three Walkers symbolically using three beer bottles. He later faces him just off the shores of Demonreach, where Dresden and the Wild Hunt try to foil Sharkface's plot to free the Naagloshii and the Dark Gods buried underneath the island with an army of Outsiders.