"He was the designer of the MS-DOS executable file format, and the headers of that file format start with his initials: the ASCII characters 'MZ' (0x4D, 0x5A)"
Ref: https://en.wikipedia.org/wiki/Mark_Zbikowski
Considering he "only" joined MS in 1981 (which was found in 1975?), I'm surprised no more people between him and Bill stayed at MS.
Also shouldn't Paul Allen still count (from Wikipedia, "Allen resigned from his position on the Microsoft board of directors on November 9, 2000, but he remained as a senior strategy advisor to the company's executives.")?
Edit: wow, never knew he worked at Valve, too.