Nameless characters are just characters that do not have a name on top of their speech bubbles when they talk.
To remove the name:
Go to the character settings in the portal.
Click on the character you want to edit.
Delete the display name at the top.
That's it! The name will be removed from the speech bubble.
Stranger to Friend?
If you want a character to appear nameless at first and then show their name later in the story, you will have to create two of the same character: one with a display name and one without the display name. Use the nameless character first and when you want to show their name, switch out the nameless character with the other character that has the name.