You can also just play it, since it's f2p. And you can unlock quite a few characters without paying a single cent. Besides the MC, I've got Amber, Lisa, Kaeya, Barbara, Noelle, Bennett, Diluc and Xiangling simply by playing the game. That's far from being all of them and I've got like a thousand Barbaras from the damn RNG, but I don't think that's too bad for paying literally $0.00.
If you just enjoy the r34, that's fine, but it's not like they force you to whale out your neetbucks every week just to be able to play.