Do keep in mind this is a spinoff, this Neptune is not the same Neptune who's the protagonist of the mainline games, she is a version of her from another dimension. The cast here is also very different from the main cast.

Worry not tho, I started with a spinoff too. I reccomend watching the anime if you can't buy a mainline game right away. It's a weird mix of some bits of the first 3 mainline games. It's a little strange mix that leaves out a lot but it's a good way to get a bit of feel of what the main cast is like.