DI Greg Lestrade

Detective Inspector Greg Lestrade is one of Sherlock's colleagues and friends even though they don't get on sometimes in the last episode Sherlock considered Lestrade as his friend. he is the one who tells Sherlock about cases they need him to solve and was the last one to think Sherlock was a fake.

History
Detective Inspector Greg Lestrade (Rupert Graves) works for Scotland Yard and directly with Sherlock. He has a reluctant respect for Sherlock and often defends him from the other police officers' animosity. He is often frustrated by Sherlock's cryptic deductions and withholding of evidence, but believes that he is a good man (hoping that one day, he can overcome his poorer qualities). Proven in "The Hounds of Baskerville", Sherlock had absolutely no idea what Lestrade's first name was; he gave a sarcastic quip in response to Sherlock's surprise. In "The Reichenbach Fall", Lestrade agrees to bring Sherlock in for questioning and it becomes apparent that Lestrade's superiors were not aware of Sherlock's involvement in cases. Unlike Donovan and Anderson, Lestrade had complete faith that Sherlock was innocent of any wrongdoing, proving just how much he trusted him.

In "A Study in Pink", he mentions that he is in an on-again-off-again relationship with his ex-wife. In "A Scandal in Belgravia", Lestrade says he and his wife have settled their problems, but Sherlock flatly informs him that she is (as of Christmas that year) sleeping with a P.E. teacher.

Although Lestrade is ranked as 'DI', i.e. 'Detective Inspector', he is referred to in scenes as just 'Inspector', possibly as Lestrade in the original novels was ranked as Inspector (the rank of 'DI' not having been created at that time.