Randolph Carter is considerably the most important, and most definite, main character of many of Lovecraft's Dream-cycle works. Randolph Carter was possibly an alter-ego of Lovecraft, and that many of Lovecraft's dream-works were indeed nightmares he had in the past.
It seems that he lived a life quite similiar to Lovecraft. Portrayed as being one that quite often wrote unoticed literature, and was seemingly regularly depressed.
In a backstory for The League of Extraordinary Gentlemen, Alan Moore suggests that he is related to John Carter of Mars.