A main character is apart of the Social Dialog feature. For example, this is where you can join or assemble teams, access the friends list, search for other characters, and you can track your friends from different worlds depending on what they agree to. You can also configure your Friends. You can select your friends into different groups Close Friends, Friends, and Contacts. You can see their online status, their last login, account status, loyalty title, other characters, badges etc. A lot of players don't use this feature but I believe it's useful to play around with. You can tell who your main character is by the Green M that appears near their name on your characters list either in client or on the website once logged on.
You can sell your main character through the character auctions on Char Bazaar, but you cannot use the Social Dialog feature until you set up a new main character! About changing the main characters: If you don't have a voucher or can't select one, you can purchase a main character change in the Store for 250 Tibia Coins. Please note that you cannot select characters that are hidden, scheduled for deletion, or Tournament characters. Some reasons why you would want to change your main character:
- You cannot delete your main character. A way to go around this is to select another main character.
- You cannot use Social Dialog.
- You cannot hide your main character.
Sources:
June 2019: Friend Lists, Badges and Titles
June 2019: Login with Email and Selection of Main Character
FAQ: Can you sell your main character?
Tibia Manual: Social Dialog