Hoshino does not collect, store or share any personal data.
Everything you do in the app, the words you search, the lists you make, your study progress and any text you paste into the reader, stays on your device. Nothing is sent to us. There is no account and nothing to sign in to.
When the app checks for updates it sends the operating system, the app version and a random identifier created for this installation to Expo's update service (expo.dev), so that the right update can be delivered. That identifier is not linked to you or to anything you do in the app, and it is the only thing the app ever sends anywhere.
The app asks for no permissions beyond network access for those update checks.
If you delete the app, all of its data goes with it.
The dictionary is built from JMdict and KANJIDIC2 (Electronic Dictionary Research and Development Group, CC BY-SA 4.0), Tatoeba (CC BY 2.0 FR), yomitan-jlpt-vocab (CC BY) and kanji-data (MIT). The full acknowledgements are in the app under Settings → About and sources.