does anyone know what the � character means and what character format it comes in?
I sometimes have this character in my logfile.log (ASCII format) file.
For the benefit of search engines, the symbol we are talking about here (�) is a question-mark inside a rhombus. (And a rhombus is colloquially known as a diamond.)
� is a symbol, or perhaps a font glyph, but strictly speaking, not a character. It is a general-purpose symbol which is used by many fonts to indicate that they do not actually have a font glyph for a certain character that you are trying to print. The font just maps to that font glyph all characters for which it does not have a glyph, so that we humans can take notice that something is wrong and in need of troubleshooting.
So, bottom line is, by just looking at that � we cannot tell which character it is.
However, we can find out more by looking deeper into the text file.
The proper way of doing this is to obtain a hex dump of that text file. In the good old days, every programmer had a hex dump utility lying around somewhere, but nowadays with everyone fumbling data in text format, hex dump utilities have fallen out of grace, so you might not have one. Luckily, the Great Interwebz can come to the rescue:
Copy the character you are interested in, and paste it into a tool that converts text to hexadecimal, for example something like https://online-toolz.com/tools/text-hex-convertor.php (ignore the bad English, and fact that it is written in PHP; programmers come from all over the world, and programmers do make bad life choices.)
For the particular character that you included in this post, the text-to-hex converter gives efbfbd. This bit pattern makes no sense, and it probably means that some corrupt data or uninitialized memory was written into your log file.
Also do note that you might be thinking of your log file as being ASCII-encoded, but it may not necessarily be so; it may have a UTF-8 encoding, which is identical to ASCII for the first 126 characters, which are the most commonly used ones. At any rate, the utility that you are using to view your log file is in all likelihood unicode-aware, so it is probably interpreting your log-file as UTF-8, but this does not make any difference because efbfbd is nonsense either in UTF-8 or in ASCII.