Thanks for playing! There's not really a main girl - I plan to, when the story eventually wraps up, have endgame content for every girl. Nicki does play prominently as the story goes on (as do all the girls, really), though you're not required to romance her.
I will say this - with Chapter Three nearly done being written (still chasing down some coding bugs and polishing dialogue) I decided to go back through Chapter One and make notes on what to change, in the likely event I remaster it. And actually, I kind of see why people might have gripes with Nicki. Some of this was due to having a limited number of renders to work with. Some of it was me getting used to the format of writing for VN. Some of it's because the story was originally going to be a one-and-done, but I saw a way to continue it.
So, a revamped Chapter One will likely do a lot to make Nicki more sympathetic.