One thing you should focus on with your character is making him very good at assessments and skill based declarations, that is something that any Sherlock Holmes based character should use up to the point of abuse.

I came up with a similar idea to this when thinking of potential characters. Namely that a summer court noble created a Sherlock Holmes like illusion to mess with the head of some poor mortal. Due to the prominence of Sherlock Holmes in mortal culture the belief of all of the readers over the years centered itself on the simulacrum of Sherlock and allowed him to attain independence of the fairy that created him. The character would have had a mix of deceit and detection based powers due to his nature as a living illusion. Also his high concept would have been nicely alliterative, Seelie Sherlock Simulacrum.