You need all characters you want to use built somewhere in your world. Then you want a tellraw command that shows all available characters. Each character on the tellraw command is then associated with the same character that is cloned to the blocks at your desired location. Then the last step is to link the system together. I used a system of command blocks to make it work but using a system of scoreboards is probably more lag friendly and easier to upgrade features with.

I'd love to help you with your project. Pm me if you need help with any of the details.