An Account's Main Character is the character chosen by the player to "represent" the account. Every Tibia Account must have one Main Character, and this character may not be hidden. Any Character can be set as the main character, however, regardless of its level or if it's the highest level of the account.
In order to switch an account's Main Character, you have to purchase a Main Character Change on the Store for 250 
Tibia Coins. Moreover, if you create a new character, you can also set it as main character for your account.
The Main Character setting was implemented on July 02, 2019, as a preparation for the launch of the Friend List.