Ella Thompson is the therapist that John Watson sees after he is shot during the war in Afghanistan, where he is operating as an Army doctor for the 5th Northumberland Fusiliers. She believes, correctly, that John's heavy limp is psychosomatic. She is not named in the series, but her name can be found in John's blog.
She is first seen giving John Watson a therapy session. She notes that he "still has trust issues" and advises him to write his day-to-day experiences in a blog in order to better adjust to civilian life. She thinks that his limp is due to John being upset by the horrors he faced during the war in Afghanistan; Mycroft Holmes disagrees, thinking that John really just misses the action, and advises John to fire his therapist.
Ella is seen again after John has come to see her for the first time in eighteen months due to him not being able to come to terms with Sherlock's supposed death and the "truth" that he was a fake. In "The Sign of Three" John tells James Sholto that he's having occasional therapy sessions, for "sort of a top-up".
Ella is seen next at the end of Series four's The Six Thatchers where she is seen giving therapy to somebody about the events surrounding Mary Watson's death. Due to angle this session is shot and that the man doesn't speak, we are lead to believe this is John but it is revealed to be Sherlock.