Doctor Samuel Ferringo, most often referred to as Dr. Feringo, is a physician residing in the village of Ransylva. He is an older man, described as having the intelligent look of a distinguished old gentleman, and he serves the community as its local doctor. In his younger days, he had a different occupation as an Arachni-man Hunter, which indicates a past filled with dangerous work against the kinds of monstrous creatures that roam the frontier. His life has been deeply intertwined with the Lang family for many years. He was a friend to Mr. Lang, the children's father, and was even the one who personally delivered both Doris and Dan Lang into the world. Following the death of their parents, he assumed a caring, fatherly role in their lives, looking out for their welfare on a daily basis.

Dr. Feringo is portrayed as a fundamentally good-natured and intelligent man whose primary motivation is the safety and well-being of Doris and Dan. When Doris is bitten by the vampire Count Magnus Lee and plans to resist becoming his bride, Feringo is acutely aware of the danger she faces. He distrusts the enigmatic vampire hunter D, finding his appearance and demeanor threatening. However, his deep care for Doris overrides his personal misgivings. He actively supports the plan to destroy the Count, even intervening with the village mayor to argue against imprisoning Doris and to give D a chance to succeed. His role in the story is that of a supportive, if initially skeptical, ally and a paternal figure for the protagonists.

His relationship with Doris and Dan is the most significant in his life, characterized by genuine affection and a sense of responsibility. His connection to their late father, who was once his student, further emphasizes this bond. His interactions with D are marked by caution and concern, as he represents a dangerous unknown element, but he sets this aside for the greater good of the children he seeks to protect. As the story progresses, Dr. Feringo undergoes a tragic and horrifying transformation. He is turned into a vampire by Count Lee, becoming a creature he once worked to oppose.

In his corrupted state, his personality is twisted by his new vampiric nature. He betrays his former wards, leading Doris into a trap and revealing a monstrous hunger for her, regretting that he never touched her before. He begs the Count to allow him a few drops of her blood. This fall from grace is short-lived, however, as he attempts to feed on Doris against the Count's orders. For this transgression, he is staked and killed by Count Lee’s daughter, Lamika. In his final moments, as he falls to his death, the remnants of the good man he once was resurface as he asks Doris to forgive him. As a character, his notable abilities are not overtly supernatural in his human state, but he possesses the knowledge and skill of a physician and had prior experience as a monster hunter. His legacy is that of a caring surrogate parent whose ultimate fate serves as a grim illustration of the tragic and corrupting power that vampires hold over the world of Vampire Hunter D.