Krogstad explains to Nora that forging her father's signature to obtain a loan was illegal. He then makes clear the difference between Nora and himself: While his social position was destroyed by his forgery, hers is still intact.
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.
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.
In the past, he too committed the crime of forgery, an act that he did not go to prison for but that nonetheless ruined his reputation and made it extremely difficult to find a respectable job.
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.
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).
This man, Nils Krogstad, is the person from whom Nora has borrowed her money. It is then revealed that she forged her father's signature in order to get the money. Krogstad threatens to reveal Nora's crime and thus disgrace her and her husband unless Nora can convince her husband not to fire him.
She has never told her husband because she knows it would upset him. Unfortunately, a bitter bank employee named Nils Krogstad is the man who collects the debt payments. Knowing that Torvald is soon to be promoted, he tries using his knowledge of her forgery to blackmail Nora.
After revealing the information he can use against her, Krogstad flat out threatens Nora. At this point in the play, Krogstad is not only seeking to restore his reputation, but wants revenge if he cannot keep his job at the bank.
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.
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.
Krogstad explains that his reputation at the bank, destroyed by an indiscretion in the distant past, is extremely important to him and to his social respectability. He threatens again to reveal Nora's secret. Nora then vehemently responds that he can do his worst.
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 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.
Krogstad enters and announces that he has been fired. He says that the conflicts among Nora, himself, and Torvald could be solved if Torvald would promote him to a better job in the bank. Nora objects, saying that her husband must never know anything about her contract with Krogstad.
Nora plans to appeal to Torvald's friend, Dr Rank, to save Krogstad's job, but Dr Rank has bigger issues on his mind – he is dying, and he is in love with Nora. She cannot ask him to intervene for Krogstad and as a result, he is fired.
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.
When Krogstad reunites with Christine, he is fully redeemed. If not for Christine's dissuasion, he would've even demanded his letter back unopened, so that Torvald would've never known anything.
Krogstad's decision to not expose Nora's secret is not convincing, because as a character, he is very manipulative. Krogstad's shift from villainy to generosity is not adequately motivated, because he achieves his goals through threats and black mailing.
What does Krogstad fear will happen if Mrs. Linde is hired? He will be fired.
What does Krogstad's second letter say? He relieves Nora of her debt.
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 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 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.
He does not like Krogstad because he feels Krogstad is a threat to his reputation in his new position at the bank. Torvald does not approve of Krogstad's past, but he feels especially wary of Krogstad's overly familiar mannerisms at the bank.