If you haven't noticed yet, Deemo is currently free for a limited time on the iOS App Store. Deemo is a piano focused rhythm game with a heart warming simple story. Also Brave Frontier (Japan) is having a collaboration event with Deemo featuring an exclusive dungeon and the Deemo character. Here's a screenshot taken by a player.
In my opinion, this is one of the best rhythm games I have ever played. I also love the story and bought all the DLC packs way back before the game became free for a limited time. Anyway, if you somehow miss the free event, you can get the game for $1.99 or ¥ 200 without the DLC.
Official Website: http://www.rayark.com/g/deemo/