Del Gruner is a serial killer who was responsible for raping and maiming Kitty Winter. He is a senior vice-president for Leda, a large insurance company.
|Gruner hires Joan Watson to work at Leda in the investigations unit and shows her to her office on her first day. She leaves after Sherlock informs her that Kitty's kidnapper is in NYC. Later, Watson asks for Gruner's help accessing an insurance database to look for a suspect with a rare blood disorder. After a suspect Watson and Sherlock Holmes were pursuing is killed, Gruner calls Watson who thanks him for the leeway to miss her first day at work. He gives her another day off and wishes her well. Kitty is in Watson's apartment and hears Gruner's voice on her phone. She listens to a video of Gruner and sobbing, says it was Gruner who raped and tortured her. ("The Illustrious Client")
|Holmes believes that Gruner hired Watson to gather intel on Kitty. Gruner is seen listening to Watson and Holmes' conversation through her company phone. Gruner calls Watson and fires her. Gruner is brought to the 11th Precinct and questioned. He was in London when Kitty was tortured and has broken fingers, like those that Kitty damaged. However, there is no evidence to hold him. Later at night, Kitty is waiting for Gruner outside his home. Watson goes to a charitable benefit that Gruner is on the board of. She indicates they are close to finding evidence of his crimes and in private, he grabs her arm and threatens her. In an abandoned warehouse, Gruner is tied to a chair while Kitty heats up an iron poker. They are interrupted by Holmes and Kitty knocks him out. She leaves to talk to Holmes and when she returns, she says that she isn't going to kill him. Gruner is found by police with his face badly burned. Pictures of his past victims were found at his home. ("The One That Got Away")
- Del Gruner's character is based on Baron Adelbert Gruner from Conan Doyle's story The Adventure of the Illustrious Client. In that story, Gruner's face is also disfigured by acid by Kitty Winter.