Samuel Clemens' own Missouri boyhood provided the inspiration for many of his characters. Both Tom and Huck are sketches based upon people Twain knew, for sure. In addition, many of the matronly women figures in his literature are modeled after female townsfolk that he knew growing up. Many great writers model their characters after actual people they have encountered, and Twain is one of those who uses this practice.
Other canonized authors who have "borrowed" identities include Poe, Chaucer, and Jack London. In each case, characters in stories are based upon the physical or personality descriptions of personal acquaintances of the author.