Interestingly enough, Capcom later released an “Offline” version which can be purchased as a one-time expense and played without a server. A reader reports that this version allows you to earn characters through in-game mechanisms, including purchasing them with in-game currency and earning them in events that run automatically or are manually activated.
I will probably someday be able to play this (at least the Offline version), so I am likely to expand upon this page in the future.
From what I understand, Capcom developed the original Japanese version of this game, but the version released in English-language markets was published by NebulaJoy. As of the time of this writing, it seems the NebulaJoy version is going to be discontinued soon; however, it appears that the Offline version (even the English version) was published by Capcom and presumably will continue to be available.