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You can associate your character with something else, thats easier to remember. For 7 you could use the 7 colours of the rainbow or the 7 weekdays. This is only appropriate for some storys though ,...
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Source: https://writers.stackexchange.com/a/45194 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
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You can associate your character with something else, thats easier to remember. For 7 you could use the 7 colours of the rainbow or the 7 weekdays. This is only appropriate for some storys though ,probably fantasy. Because the characters are either linked by destiny or some organization or a preordained plan, or because they all found connected artifacts... As said, not applicable to all Storys, but when you say "they have to be introduced in the first chapter", I get that feeling. Then you have the advantage that you "introduced" all of them, but you don't have to reveal anything about them if you don't want.