Throughout the play A Doll's House the character of Nils Krostad presents himself as the archetypal antagonist who threatens the happiness and safety of Nora. He is cunning, self-serving, and obviously versed in shady practices involving money. It is this latter fact that gets him in trouble at the bank where he works, and where he is bound to become an employee of Nora's husband. Knowing that his post at the bank is in danger, Krogstad utilizes Nora as his vehicle to convince Torvald to let Krogstad keep his job.
However, he does this through blackmailing Nora and threatening her to disclose a past business transaction in which Nora requested a loan from Krogstad to fund a trip that would improve Torvald's once ill-health. This type of transaction is considered an indiscretion in Nora's society, for which any disclosure of it could result in the social ruin of Nora, and in the end of her marriage.
After Nora refuses to help Krogstad, he writes down the disclosure of his business with Nora and boldly places the document in the Helmers' mailbox, knowing that Torvald will be the first to open his own correspondence. However, it is Christine Linde who changes his mind.
Shortly after placing the letter, Krogstad and Christine have a conversation where it is finally exposed that the two had a relationship some time ago. Moreover, it is also known that the relationship was serious, and that Linde was the cause of the break-up.
Krogstad: Was there anything else to understand except what was obvious to all the world--a heartless woman jilts a man when a more lucrative chance turns up?
This, in turn, left Krogstad quite heartbroken. It is evident from the information that this break up is perhaps what causes Krogstad's bad behavior, and what prompts him to lead a wretched life.
Krogstad [more gently]. When I lost you, it was as if all the solid ground went from under my feet. Look at me now--I am a shipwrecked man clinging to a bit of wreckage.
Similarly, Mrs. Linde confesses how her life has not been good either. She is now a widow with no children and no money. Even worse so, Christine is not shy to admit that she is miserable and all that she wants is a purpose to live; someone with whom she can age, of whom she can take care.
It is this disclosure in Act III what surprises and pleases Krogstad to the point of repentance for all the actions of his past. Obviously Krogstad's loss of Christine had detoured his life forever, until this very moment.
In terms of style, this sudden change in Krogstad's behavior is a rather abrupt end to this part of the plot. Yet, the focus of the play is not to see the redemption or destruction of Krogstad as a character. Rather, Ibsen focuses back on Nora's secrets, her limited knowledge of herself, and her problem at hand.
This reduces Krogstad to being just one of Nora's many obstacles to conquer until the moment that she finally gets to know who she is, and what is her real role as a woman. For this reason, we can conclude that, while love may have changed Krogstad's ways, his change does not preclude that Nora's ending will also be a happy one. On the contrary, this shows that Nora's happiness, or at least her life, does not depend on anybody or anything but her own will