Since the primary function of pretty-print is to format code, shouldn't
it be defined in terms of write rather than display?  For consistency
with ~& it may actually make sense to define the minimal pretty-print to
be something like

  (define (pretty-print obj port)
    (newline port)
    (write obj port)
    (newline port))

Should ~C display the character, write the character, or display the
name of the character as in CL?  ~A is for display and ~S for write, so
I would suggest the latter (currently it's the equivalent of ~A).

--
Alex