"Shelly Godfrey" is the pseudonym adopted by an incarnation of Number Six that arrives on board the Galactica with a mission to apparently expose Gaius Baltar as a traitor (Six Degrees of Separation). Her real aim, however, is to further push Baltar to a point where his only hope of salvation appears to be his complete acceptance of the Cylon concept of "God". Indeed, once he has reached this point, she rapidly goes into hiding (possibly unwittingly helped by Sharon Valerii on the Galactica?) and later probably removes herself from the ship - or destroys herself.
During her time on the Galactica, "Godfrey" very much reveals her underlying nature is very similar to that of her "sisters" as she attempts to seduce William Adama.
Her appearance on the Galactica gives the clearest indication of all that Baltar's Six is an independent intelligence implanted in his head - and that, for all its statements to the contrary - it is somehow in contact with other humano-Cylons within the fleet (or at least in communication with any of its "sisters" which may be in the fleet). The appearance of "Shelly Godfrey" and the "disappearance" of Six from Baltar's mind is otherwise too coincidental to be believable.