Unicode Pad is an Android App that enables you to input every character in Unicode. You can download it from Google Play: https://play.google.com/store/apps/details?id=jp.ddo.hotmist.unicodepad
Any contributions are welcome. They will be reviewed by @Ryosuke839 and may be included in the next release.
Currently, I'm looking for translation contributions especially. https://github.com/Ryosuke839/UnicodePad/blob/master/app/src/main/res/values/strings.xml