May Holmes is the deceased mother of Sherlock & Mycroft Holmes and the ex-wife of Morland Holmes. May and Morland divorced when Sherlock was young, and, having signed a prenup, Morland was given full custody while May was given little visitation and a small allowance.
May was mentioned by Sherlock in the episode Who Is That Masked Man? in season 4. In that episode, it was revealed that May was a drug addict, like her son. She is mentioned on numerous occasions throughout the series, mainly by Sherlock.