I'm afraid the game is only at version 0.1 at the moment, so you can't go too deep into the relationship with the girls yet.
The main story is very important because, depending on your decisions, you can share exclusive moments with them or block their romantic routes. But, where you can really discover the stories of the girls and build relationships with them (or watch them fall in love with other girls) is with the personal and rival events, which you can activate at your convenience.
In version 0.1 only Nana's personal quest (a hidden character in the game) or Nana's rival quest with Katsune is available. But version 0.2 will be released soon and will include each girl's personal quest and rival!
If you have any other questions, please let me know!