Quiz RPG has now hit 29 million downloads in Japan:
http://www.serkantoto.com/2014/08/22/colopls-white-cat-project-3668/
More interestingly, we have a new champion for fastest ever downloaded mobile game (at least to 7 million) in Japan. That would be Colopl's newest game White Cat Project.
What's more notable about this than usual is it actually looks more like a traditional game than normal. It's essentially an NDS-ish action RPG (or mid-2000's Korean f2p action RPG MMO if you prefer).
Here's a few videos to get the idea across based on how much time you have:
https://www.youtube.com/watch?v=2rpM017TGAE (26 seconds)
https://www.youtube.com/watch?v=6Oq--A6NTkI (1m 52s)
Here's an in depth article about it:
http://www.serkantoto.com/2014/07/26/shiro-neko-project-colopl-3668/
Summary:
-Touch based action RPG you play with one finger, a la just using the stylus in say Phantom Hourglass.
-Up to four people can play in co-op together.
-You create your own character.
-There's a story as voice acted by many famous voice actors.
-You build a town that lets you upgrade your character and help advance the plot along with completing missions.
And finally some screenshots to help explain it a bit better if you use youtube where you are:
It's currently the #5 top grossing app in Japan on both iOS and Android. I've only included the iOS chart below in the name of space saving and since it's the more competitive market.