Nils Krogstad is, at least at the beginning, the antagonist of the play. Known to the other characters as unscrupulous and dishonest, he blackmails Nora, who borrowed money from him with a forged signature, after learning that he is being fired from his job at the bank.
Then the audience learns that Krogstad, a co-worker of her husband Torvald, has the power to blackmail Nora. She forged the signature of her dead father when she obtained a loan from him, unbeknownst to her husband. Now, Krogstad wants to secure his position at the bank.
What does Krogstad threaten Nora with? What does he do as he leaves her house? He threatens her by saying that he could tell an outsider about her crimes but he will keep her predicament between himself, Nora, and Torvald if he gets a better job at the bank. Krogstad also says that he controls her final reputation.
Unlike Torvald, who seems to desire respect for selfish reasons, Krogstad desires it for his family's sake. Like Nora, Krogstad is a person who has been wronged by society, and both Nora and Krogstad have committed the same crime: forgery of signatures.
Nora goes to greet him and then, very prettily, coaxes her husband once more to allow Krogstad to keep his position in the bank. Nora says she is afraid he might write malicious slander about Torvald in the newspapers, threatening his new position just as her father had once been threatened.
After Nora rather easily admits to her forgery, we learn that Krogstad's bad standing in society, the cause of his moral disease, results from committing the same crime. He tells her that “what I once did was nothing more, and nothing worse, and it destroyed me” (Ibsen 166).
It turns out that he is the person Nora borrowed money from. He's got a lot of power over Nora, because apparently she forged her father's signature after he was dead in order to get the loan. Krogstad threatens to expose Nora's crime if he loses his job. After he leaves, Nora freaks out.
She asks that Torvald give Mrs. Linde somebody else's job. Torvald, outraged, says he will not act on Nora's “thoughtless promise” to Krogstad. Nora replies that she is terrified of Krogstad, who can do Torvald harm because he writes in all the “nastiest” papers.
Krogstad is an easily misunderstood character who despite his previous help during a period of crisis is detested by Nora, cynically lampooned by Rank, not given an opportunity by Torvald as a former friend of his, only to be understood by Mrs Linde.
He found out that “Nora forged her father's name on the bond she used to secure the money”. Krogstad wanted to take advantage of that situation. He asked her to convince her husband to retain him. Krogstad told Nora that if she doesn't agree on that deal, he will tell the authorities about the forgery.
Linde once had romantic relations with Krogstad but broke them off in order to marry Mr. Linde, who had more money. Mrs. Linde says that she felt the marriage was necessary for the sake of her brothers and mother but regrets having ignored her heart, which told her to stay with Krogstad.
While Krogstad initially does try to blackmail Nora, he has a more complicated backstory and eventually relents. Krogstad's motives and circumstances are of no interest to Nora and Torvald at this point in the play; how one appears to others is all that matters.
In his time, a man is no longer respected in society without having a good job from which he can earn money. Once he feels that his job is in jeopardy, Krogstad finds no choice except blackmailing Nora. The financial variable that pushes Krogstad to commit an illegal crime is the need to save his motherless children.
What effect does Krogstad's blackmail have on Nora? It breaks her will to achieve independence. It deepens her love for Torvald. It makes her aware of her true feelings.
Nora asks what Krogstad did to warrant his bad reputation. Torvald responds that he forged signatures. Nora asks what his motives were in the matter. Torvald says he would never condemn a man for one indiscretion, but the real problem with Krogstad was that he refused to admit what he had done and take his punishment.
Bitterly Krogstad reproaches Christine for renouncing their betrothal, years ago, sacrificing him in order to marry a man better able to support her and her family.
Torvald's decision to fire Krogstad stems ultimately from the fact that he feels threatened and offended by Krogstad's failure to pay him the proper respect. Torvald is very conscious of other people's perceptions of him and of his standing in the community.
What is this? Krogstad has a criminal record of having committed forgery, and is thus extremely protective about his position in the bank, as he finds it extremely important to hold a respectable position in the bank in order to redeem himself.
Torvald is a law-abiding do-gooder while Krogstad is a shady blackmailer. When Torvald is willing to submit to Krogstad's blackmail, we see that all his talk of being honest and forthright was really just talk. Torvald is ultimately just concerned with appearances.
Krogstad is different from Torvald because he is not as well off economically and socially and this leads him to try an extremer form of manipulation. Krogstad is desperate to keep his job at the bank and tries to get Nora to secure the job with her husband through blackmail.
For the last time, Krogstad asks Nora to help him keep his post. If necessary, he says, he would produce the forged bond in court. His parting words frighten Nora, and she tries to distract herself by considering her Christmas decorations. Interrupting her thoughts, Torvald comes to ask what Krogstad wanted.
The main message of A Doll's House seems to be that a true (read: good) marriage is a joining of equals. The play centers on the dissolution of a marriage that doesn't meet these standards.
At first, Nora's interaction with Dr. Rank is similarly manipulative. When she flirts with him by showing her stockings, it seems that she hopes to entice Dr. Rank and then persuade him to speak to Torvald about keeping Krogstad on at the bank.
Later in the play it is revealed that he was once in love with Kristine Linde, who ended up marrying another man in order to have enough money to support her dying mother and young brothers. This left Krogstad lost and embittered, unhappy in his own marriage, and is presented as the reason behind his moral corruption.
Even though she loved Krogstad, Mrs. Linde believed she had to marry someone with money so that she could take care of her family. She sacrificed her own happiness and reputation in order to fulfill her duties to her family.