This app was mentioned in 12 comments, with an average of 1.83 upvotes
And what exactly is that app called? I found something
https://play.google.com/store/apps/details?id=jp.co.ideaf.neptune.nepkamijigenapp&hl=ja
but it says last update was 2018年12月20日, probably wrong one?
QooApp will not work. They explained that it requires verification from the play store anyway, so downloading the APK is of no use.
You can always download from rhe Japanese Play Store. Just grab a Japanese VPN, go onto a PC, and go to Google payments center to change your billing address to somewhere in Japan. Then, download the app using this link and use your credit card or get a Japanese gift card to buy the in-app purchases. I think you can switch your region back to whatever it was after downloading the app, since it will then be already registered in your inventory.
What... A simple Google search led me to the US and AU versions of Apple's App Store, containing Hyperdimension Neptunia the App. Play Store only has the Japanese version
まいにちコンパイルハート. Don't have an iPhone so I gave the Android link.
神次元アプリ ネプテューヌ (Hyperdimension Application Neptune)
Available on:
Google Play and iTunes
Available on Google Play and App Store.
English Guide to unlocking secret events are found here.