Every girl you do not have yet is in the epic pachinko.
Last event you were guaranteed an event girl if you did a 10x epic pachinko, but for some reason they changed it this event.
So now you can get any girl you don't have already.

As for it being a ripoff;
Oh no, they have a system in play to get some money.
Not everything is free, wah wah wah.
I am sorry to have to say this, but you are whining like a baby.
Without that system, the game wouldn't be here in the first place, and you wouldn't have anything to whine about.
You don't have to pay 2 kobans to collect all the income, you can just collect it all for free one at a time.
That will require you to invest a minute.
There is a system in play to give you some kobans for free, but obviously you will NOT get enough to get everything possible in the game.
I'd rather have the kobans be more expensive and make pachinko only girls a bit more expensive than that the game is unplayable because you have to pay to get anywhere like with stupid games like pussy saga.