There is a problem as stories progress (especially in never-ending web novels) where the cast of characters keep growing. Some webnovels I follow are in the hundreds!
Is there a technique for authors to keep track of their cast of characters?
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The only technique there is really is keeping some sort of "character sheet" for each character.
If you can keep them all in your mind, that's great, but I guess you wouldn't have been asking the question if it were that easy for you. Otherwise, people use different platforms. I like OneNote, I've seen others use Scrivener, or physical folders with actual paper, some do just keep it all in their heads - whatever works for each writer.
You want to keep track of stuff related to each character: appearance, personal history, goals, views they express. The big stuff, like what kind of person they are - you're not going to forget that as a writer. But little details like eye colour or a food allergy - those are easy to mess up. They're not really important, so you write something in one scene, then forget about it and write something different next time the issue comes up. As an example, the character Thomas Raith in Jim Butcher's Dresden Files series is just under 6 feet in some books, a little over 6 feet in other books. When this was pointed out to the author, the next book mentioned how the character wears shoes with whatever heels are fashionable at the moment, so the first-person narrator isn't really sure about his real height.
The way you avoid this is you try to double-check such things on the character sheet you're keeping.
To clarify, you don't need to create all this stuff in advance. But as you're writing, make notes of what you're writing about the character, so you don't contradict yourself later. (And of course, if there's something you don't remember, such as whether you've already killed a particular minor character, you can Ctrl+f your file.)
Time for me to recommend one of my favorite webservices (free) again! Archivos!
Why I favor this is it allows you to define and keep track of relationships between everyone, and also to events (who was at the battle of tweedledee?) and places (was the doctor on Mars at the start of the battle, or in the command ship?)
This is link showing some works using it: https://blog.archivos.digital/archivos-story-samples/
Basically, everything is a Story Element.
You can choose from the following list to define the Type of Story Element you’re creating:
Person* – from protagonists to the smallest walk-on role
Region* – worlds, continents, countries, counties, mountain ranges, forests, etc.
Location* – towns, buildings, landmarks, ruins, etc.
Organization – governments, guilds, religions, cabals, corporations, etc.
Item – relics, artifacts, unique tools, cars, ships, etc.
Event* – battles, treaties, plagues, births, deaths, graduations, etc.
Culture– ethnicity, as well as speculative cultures (elves, dwarves, giants, aliens, etc.)
Discipline – magic, kung-fu, cloak fighting, psionics, etc.
*These Story Elements have additional unique properties:
Person allows the Storyteller to assign an optional gender (Male, Female, Other)
Regions and Locations allow Storytellers to assign map graphics to the Element and configure it for display in the Living Map (qv)
Events allow Storytellers to assign a date, date range, and/or time to the Event Element
So you can add relationships like work ones (hierarchy) or romantic, or who is in a group, etc. Each character can have an image, and there's space for all sorts of notes, etc.
I would have a document like the one @Galastel suggests, which makes good sense for 'characterizing your characters', but I also keep a 'mindmap' with a visual representation of how all my characters are connected to one another.
I personally use 'Mindmeister.com' and have been quite satisfied with it. You can make your own rules for coloring and labeling, i.e.: Red line = family, blue line = friendship, green line = other...
You can export your mindmaps as pdfs to keep around or to send to potential readers, or the cast of a play if that is the case.
It is free to use, and they have a website + mobile apps. You create a log-in that works across these.
Personally, I have a combination of the above ideas. Where I have a list of characters with their hierarchical attributes (affiliations, relationships, living/dead, romance, etc.) which is linked to their "File" which is written up in OneNote. I also have a mastermind map (also in OneNote) to allow me to visualize these characters.
Now, if I am being truly honest. I primarily write shorter stories (working on my first novel though) and I do this for each story. However, sometimes a character gets carried over or copied in essence to the other. This allows for faster writing and smoother character development, in terms of pushing along the story, as I can take what I need at a glance and keep working.