While Alexes acts cheerful most of the time to hide his true self, Nicolai doesnt even bother. His main reasoning for this is 'If I don't care, why should they? If they don't care, then why should I?'
So it kind of works both ways.
Despite this though, he is actually quite trusting - but only if you pass his first inspection. If he decides you are harmless from the first, he will trust you and try his best for you - unless you annoy him, or his gut instincts prove stronger than his friendship with you.
He follows his instincts, and since they rarely make mistakes where people are concerned, they are the basis of his decision of whether or not he likes you.
He is also slightly naive, he learns his lessons well, no matter how hard, and nobody will ever take him by surprise by that method again - unless he's willing to be fooled.
Nicolai is a loner by nature, but he doesnt really care either way. He enjoys good fantasy novels, and a few well written mysteries also provide entertainment.
he has an extremely cold temper that is no less dangerous than Alexes', but Nicolai would rather storm off and cool down before he sees or talk to the people that made him so upset. He prefers not to lose his temper in public, because it is an extremely private matter with him, but if he cant help it, and he gets angry anyway, then he'll probably be more angry than ever, all because he couldnt conceal it.
Spoilers (highlight to read)
Nicolai was a soldier of the enemy country, Raegwyn. He had a respected rank, and his troops respected him for the most part.
His favorite weapon was a weapon native to his country, a decorative staff with a sword blade on one end, and a long, thin dagger blade on the other. The more decorative the middle was, the higher your rank, and the shorter the staff, the better you were at using it.
Nicolai had been trained in the art of war since he was a youngling, and he is the youngest soldier of his rank, which didnt sit too well with his equals and superiors. The inferiors in other troops that dont answer to him didn't like it either.
He was killed, not in an honorable warrior's death, but in disgrace, burnt to death, falsely charged with treason after he had shown respect and pity for a prisoner of war (Alexes). His superiors took the chance to get rid of him before he could climb any higher in the ranks.
Since it is generally accepted in Raegwyn that their citizens do not have their own mortals, no one thought to barricade Nicolai's soul from escaping, and escape he did, seething with the injustice but unable to prove it.
A wandering spirit gave him a special test in reply to his request, and when Nicolai passed, he earned a mortal body that had no claim to it yet.
He doesnt remember his past either, and doesnt find out until at least a few days after Alexes does... It kind of surprises the both of them.
End Spoilers
Nicolai doesnt fall into like or love easily. The only reason he got together with Alexes was because they had known each other since elementary school, and they were both somewhat lonely.
Nicolai knows perfectly well he is starting to love Arical, and as much as he doesnt like the fact, he doesnt really want to stop himself either. He senses that although he and Alexes do love each other in the best way they can, he can love Arical so much more.
He doesnt understand why he senses this, and it hurts him, whether he acts with it, or against it, because he's very loyal, and doesnt want to hurt Alexes. It is also one of the few times he's gone against his gut instincts (which are telling him to get on with it and go with Arical) and that annoys him almost constantly whenever Arical is around.