By focusing only on Nora and Torvald, the first answer ignores the fact that there are seven characters in the play. Let me list them for you and give a brief description of each one.
The Porter: appears in only one scene and doesn't figure in the action.
The Maid: She has very few lines and doesn't figure in the action.
Nora Helmer: The main character and the person around whom the action revolves. She is married to Torvald, and she is keeping a secret from him that will eventually ruin their marriage when it is discovered. She only pretends to be helpless.
Torvald Helmer: Nora's husband. He treats Nora like one of the children, constantly calling her "little squirrel" and "featherhead." He constantly fusses at her for spending money and eating sweets. He cares very much about appearances and suggests an unconventional solution to the problem when Nora's secret is revealed.
Dr. Rank: Torvald's friend and physician. He is secretly in love with Nora and treats her with more respect than her own husband does.
Christine Linde: An old friend of Nora's who is widowed and comes to Nora to ask for help getting a job. She married for money instead of for love, but she is now penniless and bitter about it.
Nils Krogstad: the antagonist of the play. He works for Torvald but knows he is going to be fired because of mistakes he has made. He has discovered Nora's secret, and in a desperate attempt to keep his job, he blackmails her into persuading Torvald not to fire him. He was the love whom Christine left behind, and they are reconciled in the end.
I hope this helps you.
You have obviously asked a very broad question, so I will only be able to respond with a general answer. The two central characters that you need to know about are Nora and Torvald Helmer, the married couple that the play focuses on.
Nora is the doll of the title, who finds herself trapped in a marriage where she is patronised and treated like a little child by her husband. However, as we see from their very first moments together on stage, this is a role that she accepts and plays very well. Her husband even goes as far as to pat her head like some adoring dog or other pet. However, we quickly realise that Nora has many thoughts and ideas that she conceals from her husband, and thus to a certain extent is a split person. She is constantly objectified and believes she is dependent upon her husband throughout the play.
Torvald seems to occupy a symbolic role in this play. As a lawyer in a patriarchal society, he embodies all of the chauvanistic assumptions of people at this time, particularly regarding the inferior and weaker position that women were deemed to occupy. As we see in his relationship with Nora, he constantly treats her like a little girl, which, we discover, is how she was treated by her father as well. He never treats her as a serious individual in her own right who is capable of making mature decisions, and thus Nora's awakening at the end of the play comes as something of a shock to him.
I have included other links below to various enotes study sections on this play, so I hope this helps you gain more of an understanding of this excellent work of literature. Good luck!