Helen Stoner's abusive stepfather and primary suspect in her sister's mysterious death. Violently confronts Holmes and Watson but ultimately meets his demise when the venomous snake he intended to use on Helen escapes and bites him in his own room.
Dr. Roylott serves as the primary antagonist who uses intimidation and lethal means to maintain control.
Dr. Roylott holds a familial authority over Helen Stoner / Julia Stoner that he abuses for control.