Flaming Durtles is a client app for the WaniKani kanji learning service created by Tofugu. Are you using WaniKani to learn Kanji? Then you can use this app to do your lessons and reviews.
Note: requires an active WaniKani account!
This is an independently developed app that is not supported or endorsed by Tofugu.
Support thread on the forums: https://community.wanikani.com/t/android-flaming-durtles-android-app-with-offline-support/38400
Open source software: https://github.com/ejplugge/com.the_tinkering.wk