Harvey is an old rival of Stemple's from Harvard. He defeated Harvey three times in mock trials while they were Law students, but has been avoiding confrontation with him ever since.
In "Moot Point", Mike wants to give Harvey a present, and not knowing what to give, he asks for Donna's help. She explains to him there's something Harvey wanted was never able to get it, which was a confrontation with Stemple. When he encounters Harvey in court, he teases him until see that he is the one who he has to defeat. As the case goes on, he says his analyst will come up with anything he wants on the stand.
Later, he begs Harvey to sign the deal, but with no success. That's when Stemple confesses he cheated in Harvard to beat Harvey. After this, the two go back to court, where Elliott invalidates Harvey's argument, later revealing the whole history about cheating in the past was a set-up to make Harvey's client look like a thief on the court. He is defeated by Harvey and Mike when they put together proof against Stemple for corporate espionage.
Stemple also appears in "Accounts Payable", where he blackmails Harvey to give him the painting in his office.