like many other cell games nowdays it's free, no advertisement but a specific currency in the game can be purchased with real money.
In this games are the diamonds:
which are used to open the gold box that could give a rare character.
common character are all 1/2 star and some 3 stars.
Bronzo five V1, V2, the other bronze, black saint, Kiki, Cassius, Jango, Pope, Roshi, and half silver saint are common character you plays always.
Marin, Algol are common, while Shaina and Misty aren't, you have to find them in the gold box that consumes 5 diamonds.
The same for the Golds saints (4/% stars), Marina and Specter.
In the game you can find diamonds, and provides at least 6 diamonds per week if you log in everyday.
The real problem that the chance to get something rare are TOO low. Imho less than 3%.
So even if you buy 30 diamonds for 2300 yen, you likely won't find anything good.
So imho the real money doesn't make the difference. It's all a matter of luck.
But there're monthly events you can win (not that diffiult) gold saints, marina, ....
So it isn't that bad.
Of course, if you just like gold saint and want to play all them immediately, you better buy Soldier' Soul.