This addon shows you the name of the main character when one of your guildmates chats using a known alt. This is done by scanning the guild roster, looking for the name of a main in the player note of an alt. Many guilds already do this just as a matter of record keeping, so this addon works right out of the box (or out of the zip file I guess!)
The names show up in parenthesis after their alts name like this:
[Bravehearth (Pitchifus)] whispers: Hey, is the download ready yet?
Also, achievement and logon/off messages are displayed like this:
Bravehearth (Pitchifus) has earned the achievement [Home Alone]
[Bravehearth (Pitchifus)] has come online
Finally, it also extends the /who command so that if you do a /who on someone's main and they're not online but they are online on an alt, you'll see a message like this:
/who Pitchifus Pitchifus is online on [Bravehearth] 0 players found
It was designed using the default chat UI and compatibility with custom chat windows is not tested. I'm confident that the achievement and logon/off message will work with any chat addon, but the feature of showing mains in communication messages (whisper, say, guild, etc.) may or may not work with other chat addons depending on their design. Please file a CurseForge ticket if you find a chat addon that doesn't appear to work and I can try to make it compatible.