I mean, Yuri is unlockable as the player character if the player finishes Mari's route (collecting all 12 Buff Cards, dropping them in the Gate Field (Nexus) and return to real world to see the ending of Mari's route). Note that Yuri has a different "Dream World" than Mari (with different Buff Cards to collect), and a different way to access it (similar to Sabitsuki from .flow), and a different "story" route (Yuri route).

The same could apply to all other characters I planned to add into my game as unlockable characters (if I plan any more of my fanmade characters / OCs to be playable in future versions of the game).

The problem is that the feature need me to tap into some persistent data plugins, to which some are paid (not free).

Anyway? Do you want me to add Yuri as an unlockable character, and want more unlockable characters after that?