Oi, it's actually important to the plot, believe it or not. Nikolai is nearly emotionless, and without Arina, the heroes wouldn't be able to defeat Dmitri in the first place. Nikolai wouldn't save Arina from getting killed by Dmitri if something wasn't going on between them, and Nikolai wouldn't kill Dmitri if he wasn't so cruel to both him and Ariana. If it were just him, he would probably get killed by Dmitri, due to not being able to stand up to him.
It sounds like a really twisted soap opera, kind of. But with demons. And yes, it's not a romance novel, so you don't have to worry about that.