Sherl is a dog suffering from amnesia, unable to remember his original name or identity. He first meets Katrielle Layton shortly after she opens her detective agency, presenting her with a puzzle to test her skills. Impressed by her solution, he asks for her help in uncovering his true past and subsequently takes up residence at the Layton Detective Agency. Katrielle names him "Sherl O. C. Kholmes" in reference to Sherlock Holmes, though he is usually called Sherl.

Sherl possesses the ability to speak, but initially only Katrielle and her assistant Ernest can understand him; others hear only barks. His sense of smell is notably poor for a dog, comparable to a human's. In the anime, Luke Triton is later revealed to understand Sherl due to his natural ability to talk with animals, expanding the circle of those who comprehend him.

His past remains largely unresolved across official media. He retains a single memory fragment involving an explosion. This fuels theories connecting him to the genetically engineered "Subject" animals from *Professor Layton and the Unwound Future*, specifically the unaccounted-for Subject 2 after that game's explosion. However, the experiment would need to have significantly extended his lifespan for this to be likely.

At the agency, Sherl serves as Katrielle's assistant. He occasionally joins her in humorous detective lore references, like her teasing suggestion he would need to relocate to "Barker Street" to run the agency independently – a nod to Sherlock Holmes' Baker Street. His personality surfaces through quirks such as singing lines from "Hound Dog" after solving puzzles.

The mystery of Sherl's origins, identity, and the reason for his amnesia remains officially unsolved by the conclusion of the series.