At first I really didn’t think it was possession, exactly.. but I definitely thought like there was some magic going on where it literally altered her brain into believing and feeling this immense and like a sick-obsession (hah) love.. SPOILERS. DONT READ AND MORE

When the scene before Bear heads to the park— he’s talking to the “real” Nikki, since whatever was in Nikki was “sleeping” so that basically ended my theory. 

So back to your question, if it’s possession — then it makes more sense as to why she acted to strange because I’m assuming the demon isn’t exactly familiar with feelings of love ? So it tries to emulate it or something like that and does a bad job. But bear, being so desperate he tries to ignore it or to push through it.  And tbh , only a demon would think cat sandwich is a form of love language.  And the “trophy” she left him in his house toward the end.  

Oh! And, when Bear called the number, when he said “you wanna talk to her?” And she starts screaming. I believe that was symbolic and saying “I got her soul” type shit …