The main "less respectable" member of society featured in the book is Huck Finn. The son of a violent, outcast drunk, Huck lives by his wits and does what he wants. He wears ragged, cast-off adult clothes, doesn't go to school or church, and is envied by the other boys for his freedom. On the downside, he has to scrounge for food and sleep where he can—such as in a cardboard box, an empty barrel, or a barn.
The mothers and guardians, including Aunt Polly, of the respectable boys all forbid them to associate with Huck. Naturally, that, with Huck's enviable freedom and the allure of a boy who does anything he wants, makes Huck a boy the others want to play with. Tom, for example, will play with him whenever he can—but notably, Tom has also internalized the town's social code: he will find excuses not to be seen in town with Huck. Huck is Tom's avenue to a variety of superstitious information and exciting adventures he might not be able to embark on with a more respectable boy—but Tom doesn't flaunt the relationship.
Other less reputable or downright disreputable characters include the evil and murderous Injun Joe and his kinder-hearted sidekick Muff Potter. The relationship between these two men and Dr. Robinson is one in which the more respectable party uses the lesser men. Dr. Robinson, though quite respectable to the townspeople, gets Joe and Muff to dig up Hoss William's body so he can experiment on it—an immoral and illicit thing to do.
In the cases of both respectable Tom and respectable Dr. Robinson, we see relationships with "less respectable" folk that fulfill a need of the respectable party (as well as the lesser party) and that are kept more or less quiet—neither Tom or Dr. Robinson is about to publicize a relationship with someone from the wrong side of town.