There is also 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, so I am likely to expand upon this page in the future. However, I doubt I’d be able to give an online game extensive coverage since such games change so frequently, so I’ll aim for more of an overview. (The Offline one may be a different story.)