Nora Helmer is the protagonist of this play, as she is the leading character and the actions that constitute the play primarily revolve around her. As such, the dramatic question centers around her main conflict. We see Nora's husband, Torvald, treating her—essentially—like a child; for example, he monitors her consumption of sweets in order to protect her teeth, and he calls her all sorts of diminutive nicknames like his "little squirrel" and such. She even hides her errant behavior from him as a child might. The opening scene between the two of them makes us wonder if she will ever realize that she is acting and being treated like a child, and what would happen if she does: the dramatic question.
The inciting incident which so disturbs Nora's peace is the appearance of Krogstad: he reveals that Nora forged her father's name on the loan contract and blackmails her, compelling her to beg her husband to keep Krogstad in his employ.
Some might consider the antagonist of the play to be Nora's husband, Torvald; however, it is more accurate to say it is society in general. It isn't only Torvald who treats Nora like a doll; it was her father, too. And, to a certain extent, Krogstad also exploits Nora's diminished position as a woman in society. It is the patriarchal society that puts women at a social and legal disadvantage which antagonizes Nora.
Torvald is merely society's representative, and the climax of the play takes place when Nora confronts him about his always having treated her like a doll, including when she tells him that she is leaving their family to be an independent woman.