Wiki stores text as UTF-8 Unicode characters. You can enter exotic characters using your computer's keyboard extension or by entering HTML entity codes.
If you see the character you want in another document, you may be able to simply copy and paste it into wiki. Try it. Otherwise consult the list of special Unicode characters available in your computer or on the web.
Caution: Page titles are converted to simplified names that do not contain any special characters when used as internal links.
Using Your Computer's Keyboard Extension
Apple Mac OS X Special Character Menu
The exact mechanism will vary with computers. We will describe using the Mac OS X and Chrome. Other systems will be similar.
While editing text, choose the menu option Edit > Special Characters ... This brings up the character menu keyboard extension. Choose to view Code Tables and Unicode within that. Select the letter you desire. Click Insert.
Using HTML Entities
We once used HTML entities to call up special characters. We now escape them so that you can talk about HTML tags and entities but they don't become the things and characters they describe.