Follow these steps to create custom characters:
In the character selection screen, click the "+" button or the "apps" button at the top
(If you clicked the app button, choose the "Create Character" app
Fill in the details of your new character and hit save
Your character will appear at the end of all the default characters (if you want to hide all the default characters, you can do so in the settings screen)
For inspiration on creating your custom characters, you can download any character from the Personality Hub and see what descriptions they used.