Yes, you have to.
For this under development game, the payments are monthly because there are new contents updated every month. That is why it is cheap. It is different with completed games, which only need to pay once and have no added content anymore. There is no pressure to make a purchase, and you can always play the basic content for free. But for the extended version, it will require monthly payments to access. The extended version has more special interactions between each of the characters, thus a more special story. I hope you can enjoy our game. Thanks for playing!