It's interesting to me that many of my students don't know the answer to this question: Who is the antagonist? Some people argue that the story problem or the protagonist's inner struggle is the antagonist but I would argue that this is not enough and that we must embody opposition in an actual physical foe. The story problem has to be expressed through character interaction and confrontation. The inner struggle is expressed through interior thought, dialogue and action but not inter-action.
When we know our antagonist, and know him or her well, it is much easier to work on our novels. A strong, dimensional antagonist adds tension, conflict and most importantly forces the protagonist to change.
Ask yourself How does my antagonist interfere with what my protagonist character wants? How does my antagonist force my protagonist to achieve what they need? This is important because it provides obstacles to the character goal, raises the stakes and gives the reader the dopamine rush as they wonder, what will happen next? Mr. Ewell is the obvious antagonist Scout must overcome in the climax of TO KILL A MOCKINGBIRD. Boo Radley is the false antagonist at the beginning, and even Atticus can be seen as an antagonist in that he forces her to change. In my opinion, a love interest or parent or best friend antagonist is the most complex and interesting and dimensional. If they are generally a positive force in the protagonist's life you will want to add a more overt antagonist as well.