Roy Trubshaw
The Essex student who wrote the first MUD in 1978, then handed it to Richard Bartle when graduation got in the way.
Trubshaw was studying at the University of Essex, which had a DEC PDP-10. In autumn 1978 he wrote the first versions of MUD1 — a text world several people could wander through at once — initially in MACRO-10 assembly, later rewritten in BCPL. He built the engine and the database; the game content was still thin.
When he graduated in 1980, fellow student Richard Bartle took over, fleshed out the world and turned it into an actual game. The same year Essex got an experimental link to the ARPANET and outsiders started logging in. Trubshaw then left the field almost entirely, making him one of the few people to start an entire game genre without ever trying to build a career on it.