Stengel is the designer who does all the work at the successful firm of Francon and Heyer, even though the big-wigs get the credit. In fact, he's a bit like an architectural ghostwriter.
Unfortunately, Stengel has to face down that manipulative weasel Peter Keating. And good old Keating manages to manipulate Stengel right out of a job… and manipulate himself right into the Stengel's old office.
Fortunately, once Stengel is freed from Francon and Heyer, things start to improve for him. He ends up doing his own thing and eventually settles on a nice farm out in the suburbs.
Stengel is basically around to show us how corrupt and fake the world of architecture is, and how refreshing a picket-fence suburban refuge can be.