Harvey and Donna are best friends. Donna is Harvey's trusted secretary, closest friend, and confidant. She is his right hand woman and Harvey loves and trusts her more than anyone. He relies upon her and views her as essential to his being. As he told Jessica in season 2, episode 6, All In; Donna is irreplaceable. Both trust each other implicitly. No one knows Harvey better than Donna does (in fact, she probably knows him better than he knows himself), and she is protective of him.
Donna has been with Harvey since his first year at the District Attorney's office, where he worked as Assistant District Attorney and she worked as his secretary. She worked with him for two years before following him to Pearson-Hardman. She is very loyal to Harvey and is extremely loyal to him to the point where she would take a bullet for him. Donna loves Harvey more than anyone and Harvey loves Donna just as much. He adores her quick wit, her strength, her sense of humor, and he deeply respects her. Donna is as much a part of him as he is of himself.
The pair frequently indulge in jokes and witty banter, and have an inside joke involving a can opener and thumbtacks for their pre-trial ritual. Donna always listens into Harvey's conversation; in turn, Harvey always leaves the intercom on. Donna always knows what Harvey needs before he asks for it. As Harvey tells Louis Litt, he never tells Donna to do anything; she just does it. Both tend to view each other as to halves to the same whole. Over the years, they have developed an ease of comfort found only between best friends, and are very good at reading each other.
Although Harvey generally tends to keep his feelings close to the chest, he is more open and vulnerable with her than he is with anyone else. She is privy to all the details of his private life; she knows about Harvey's mother and brother and is fond of Harvey's father. She is also the one who informed Harvey of his father's death.