Nemesis (noun): someone or something that continues to oppose you and cannot easily be defeated. Thanks to Macmillan Dictionary Online for that definition.
Almost all stories have a nemesis, that bad guy or thing that our beloved main character can't seem to overcome, get away from, or defeat. One of the most famous of all nemesis of young adult books, I think, is He-Who-Must-Not-Be-Named, aka Lord Voldemort of the Harry Potter series.
The nemesis (either tangible or intangible) is something that the author needs to take into account when starting a story. In writing workshops, I teach that each story should have a story question/problem which should be answered by the end of the book. If a series is written, there can be an underlying problem throughout all of the books which would ultimately be answered in the last book - look at Deathly Hallows in the HP series. HP's problem (nemesis) is finally taken care of.
But what if the character's nemesis isn't a person? What if it is something out of their control? Both Willy Wonka and Oliver Twist use poverty as a nemesis. Think about it? Would the stories have had an impact if both characters had come from the middle-class?
Which nemesis is your favorite?