Nintendo shares dropped 9 percent on Thursday, and partner DeNA’s stock dropped 15 percent after Nintendo announced that it would wade into the mobile waters next year with social mobile app Miitomo. Based on the Mii digital avatars made popular by the Wii, Miitomo will seek to connect players through trivia, Nintendo President Tatsumi Kimishima reportedly said in a conference call with investors. The app, due to launch next March, will be a free-to-play game that will include add-ons that people can buy.