Sherlock and Irene have a close but rather strange relationship between each other. While Irene states she does not love Sherlock, the latter of the two knows she's lying.
Series 2 Edit
Series 3 Edit
Irene appears to Sherlock in his mind palace, mentally. However, Sherlock tells her to leave so he can concentrate. She is also revealed to be Sherlock's weaknesses, along with several others.