Question

Which best describes an antagonist?

the main character, often a hero
a well-developed character who is important to the story
the character who is in conflict with the protagonist, a “bad guy”
a poorly developed character

Answer 1

The character in conflict with the protagonist, but in the best stories he/she is also well developed and important to the plot.

Answer 2

The third answer, "the character who is in conflict with the protagonist, a "bad guy"". This is because that is what antagonist means, except that he/she isn't always a "bad guy".