The characters page will let you manage the characters that feature in the work. Adding existing characters, creating new characters, and assigning their roles and voice actors are all done here.
The popup after pressing the ‘Add Characters’ button will let you search for the character you want to link, which you can then select and confirm. Back on the main page, you’ll now be able to add a role and voice actor.
This will let you import every character and voice actor from another entry, which you can then modify as necessary.
Each character has the option to be set as a main, supporting, or background character. For many of these, it will be up to your own judgment.