Ancient Greek 'digamma' (ϝ) character
I believe the short answer is not all fonts have definitions for all characters. Thus it is up to the user to choose an appropriate font, i.e., with the characters they want to use. Notepad++ is designed to be a source code editor for programmers and is unlikely to have extra features to cater to scholars of ‘classic literature’.
To make it more ‘interesting’ Microsoft Windows adds an additional layer of uncertainty with a philosophy of “We will help the user by sometimes sourcing a missing character from someplace else”. (Sorry no reference for that statement, it’s based on personal observation and other web postings read over the years.) Additionally, web browsers (via HTML & CSS) have a huge variety of workarounds for supplying other/interesting/unique/rare/etc. characters.
Thank you for the explanations. I noticed that kind of discrepancy and I thought it might be useful to make it known because I thought it was related to the application, rather than the ‘internals’ of the font itself, because the same character is correctly displayed, with the same font, in other applications. (Hereby attached is a capture of the character map related to this font).
But I have no problem myself; I can manage the work I am doing using some other fonts. Thanks again.