There’s no official app just yet, the apps that are available now are made by users 
Some seem to be better liked than others, I’ve been hearing a lot of good things about TsuruKame ([iOS] Tsurukame - native app with offline lessons and reviews).
You can also just load the website from the browser, as it’s designed to look and feel good on mobile as well (personally that’s how I do my reviews when I’m not at my PC).