The novel opens with Nikolai's back-story as he waits for Arkady to return home from Petersburg. We learn that Nikolai was the sensitive child in the family. Though his father, General Kirsanov, expected him to go into the military, he broke his leg before he could enlist. As a result, he was sent to University at Petersburg, but was always viewed as something of a bad case in his family.
Like Pavel, Nikolai has a tragedy in his past. Though he was happily married, his wife died after ten years. The narrator tells us, "The blow nearly killed him and in a few weeks his hair turned grey" (1.8). After the loss of his wife, Nikolai clung to his son all the more intensely. For the first few years, he would go to Petersburg to stay with Arkady and would try to meet all of Arkady's friends. Part of the reason that he's so nervous and excited is that this is the first time that he and Arkady have been apart for an extended period of time. He's worried about letting his son get away from him right from the beginning.
When Arkady returns and introduces his arrogant young friend Bazarov, we quickly learn that Nikolai is an intensely modest man. Whereas Pavel immediately seems destined for conflict with Bazarov, Nikolai is all too ready to attempt to "understand" the ideas of the younger generation, even when they don't make sense. On the one hand, he is more empathetic and patient than his brother, but on the other, his meekness prevents him from seeing when he is in the right. When he tells Pavel, "It seems the time has come to order our coffins and cross our hands upon our breasts," it's hard not to think that he's being melodramatic (10.37).
Though Nikolai is too meek when dealing with the young men, his sensitive insight raises the question at the center of Turgenev's novel. Recalling a time that he argued with his mother, he remembers that he said to her, "Of course, you cannot understand me: we belong to two different generations" (10.121). The excuse is, in many ways, too simple. But Nikolai does raise the question of how the experience gap between one generation and the next can be closed. It's not only that he and Pavel are wiser than Arkady and Bazarov, it's also that they are hemmed in by tradition and habit: things that do not yet have a hold on the young men.
We also see Nikolai's weakness of character when he attempts to discipline the peasants on his estate. Arkady can tell that the estate is poorly managed; without the prospect of serious punishment, Nikolai doesn't know how to keep their respect. He is a liberal thinker and in favor of the emancipation. He knows what he should think and how he should behave, but at the same time his inability to command respect keeps him from adapting to the new situation. The last we hear of Nikolai is that he's attempting to appease both the gentry and the peasants, but that his opinions are "too mild for both camps" (28.9). By this point it comes as no surprise. Mildness is perhaps the overall defining aspect of Nikolai's character.
We've noted that Nikolai has a tragedy in his life, but compared to his brother Pavel he seems to have a gift for happiness. Whereas Pavel was never really united with Princess R., Nikolai had ten happy years of marriage, and a son. These days, though he's embarrassed about it, he has found a new chance for happiness in Fenichka. It's unclear exactly why things work out for Nikolai when they don't work out for Pavel, but it may have something to do with his kindness and general lack of pride. Nikolai is not a difficult person to get along with; he's happy to settle down to a quiet life and not to ask too many questions. He has his flaws, but in general he is a good father and a good husband.