Everyone using the app gets to design their own avatar called a Mii, which is created in their own image. Users send messages to others using their own Mii. While the app is free, there are in-app purchases that will allow users to buy certain clothing and accessories for their Mii. Nintendo made it clear from the very first announcement that the smartphone wouldn't be just another platform for their current line of games.
Despite the initial disappointment, interest in Miitomo has been high. And the numbers are even more impressive when you consider that the app is available only in Japan. That might change, considering the success of the launch. And what a success it has been with Nintendo's app now surpassing the emoji based messaging app Line, which has a big following in the country.
Last month, Nintendo CEO Tatsumi Kimishima made the company's fans a little happier when he said that the next release will not be a communications app, and will feature characters from Nintendo's large stable that consumers are familiar with. Nintendo has seen other game makers mint money on smartphones and doesn't want to miss out. While it could have simply converted its current games for smartphone use, the company has said that the layout of controls on a smartphone would be very different from what its loyal fans are used to, which is why it has decided to create new games instead.
Considering Nintendo's popularity in the U.S., and the grip that smartphones have over the population in the states, we would be surprised if the game maker doesn't eventually offer its mobile apps in the U.S.
source: @Miitomo_JP (translated) via Forbes