You can use Chars Keyboard in the app itself. Just tap the globe/emoji key to switch languages until you see special characters keyboard. If you want to use Chars Keyboard everywhere else in iOS, you can add a keyboard extension:
The latest version of Unicode contains about 128,000 characters. We selected the most useful and popular symbols for Chars Keyboard so it's not cluttered with all the Unicode richness. But if we'll get multiple requests for certain symbol, we'll consider adding it in the next update.
Chars Extra is bundle of additional paid functions: iCloud sync, Face ID and Touch ID protection, color schemes, and exporting notes as images. Once purchased, Chars Extra saves forever for your Apple ID account. Purchasing Chars Extra is not necessary to use the app, but it makes it more enjoyable. It also is a way to support future development of the app.
We're always trying to make Chars better. If you noticed some imperfection, please tell us.