SIOUX FALLS, S.D.—Is S. Larson for real?

People who use Citibank credit cards have wondered about that for around two decades. S. Larson sends them letters—millions of letters. S. Larson always signs these letters in the same diligent hand of a grade-school penmanship student. The words "Customer Service" appear under S. Larson's name. S. Larson has no title. S. Larson has no gender.

Some...