The oldest Rostov son, Nikolai is a hardcore patriot. He loves his emperor and embraces his military career without reservations.
Nikolai might just be the most surprising character in the whole book. He starts off as a happy-go-lucky, generally good-natured jock type. He loves hunting (the main athletic pastime of Russian aristocrats in the 19th century) and goes along with Sonya's puppy-dog love without thinking too much about it.
You know what word comes to mind? Lug. He's just a big ol' likeable lug. And he's content to coast along this way because his interests – being a cavalry officer in the army, hanging out with his buddies, his unquestioning allegiance to Emperor Alexander – pretty much fall into place without too much reflection on his part. Really, the only time in his youth that he is forced to make a decision is when he tells off his mother, swearing that he will never marry for money. Which is very easy to say at the time, since he's never experienced poverty of any kind.
And, then, just like that, his whole character flips on a dime and we find he has all these inner resources that we would never have suspected would be there. When the Rostov family goes bankrupt and Nikolai is forced to resign from the army and take care of the situation back home, he doesn't complain or defer responsibility. No, he figures things out as quickly and as best he can, taking charge of the finances, keeping his mother comfortable and happy-ish at his own expense, and even giving up his attraction to Marya because now they are no longer on an equal financial footing. (Until, of course, Marya gets him to marry her anyway.)
All very impressive and so self-sacrificing that it's pretty shocking that this is the same guy from the beginning. And yet, this personality transformation never really seems forced or strange. Why do you think that is?