You can log in as usual via the character selection after a world fusion. Characters affected by a world fusion should still be listed and accessible in the character selection, they are simply assigned to the new world.
If a character is not listed anymore it may be possible that it was deleted due to the deletion criteria.
Login with S&F account after a world fusion
Characters that were linked to an S&F account before a world fusion remain linked to it and are still accessible as usual via the character selection.
Login without S&F account after a world fusion
Characters that were not linked with an S&F account before a world fusion continue to be accessible as usual via the local character selection.
If a character is no longer present in the local character selection, even though it has not met the deletion criteria, you can import the character as usual (see below).
Import character after a world fusion
A character can be imported as usual after a world fusion. The original world can still be selected from the world selection list, and the character can be imported by entering the relevant data. Using the original character name will still work although it was renamed.