Note: this article assumes you already know the basics of training a character model focused only on one character. If not, check out CivitAI's trainer: A simple, beginner's guide to training character LoRA using it article for a detailed, but overall simple and straightforward to understand, explanation and walkthrough.
Note 2: Like this article? Please consider tipping it (or, heck, even financially supporting me at https://www.patreon.com/c/MetalChromeX and http://www.fiverr.com/s/VY34o4m) so I can continue to product high-quality models for you for free or cheap!
I thought now might be a good time to quickly whip up an article on training a character with multiple characters since not only did I just release a 12-in-1 character model with the main (female) characters from Trinity Seven, but I also have had a few people ask how to do this. From the handful of times I've done this, I've been pretty successful...after making a few tweaks. Let me provide some quick tips/a walkthrough of how I did that 12-in-1.
Firstly, let's look at what it looked like after I had finished collecting my dataset. Note that I have labeled each of these folders, like Akio Fudou (character). In multi-character models, it's practically essential you tag the name of the characters, as this will help Stable Diffusion better figure out what you're trying to accomplish.
Let's open one of these folder, Lilith Asami (character), and take a look inside. (I've blacked out a couple images that have her wearing a very spicy bathrobe just so that this article stays PG!) From here, you can see I have four more subfolders: Lilith casual outfit (outfit), Lilith magus mode (outfit), Lilith red bikini swimsuit outfit (outfit), and Lilith school uniform (outfit). The reason I make them specific like this is that, I presume, it'll make these tags more unique than, say, casual outfit or red bikini. These tags should also able to be added to other characters. For example, if you want Liselotte to rock Lilith's red bikini, the idea is that you'd add Lieselotte Sherlock (character) and Lilith red bikini swimsuit outfit (outfit) into your same prompt.
This is the basic gist for organizing your character models. The one thing I'd highly recommend to avoid bad images is going too light on your dataset. For example, I once tried doing my 6-in-1: The entire Smith family (American Dad) with 10 images for each of the six characters, which looked pretty bad. Having just 10 images per character clearly isn't enough to learn the specific character, and here is an example of what Stan Smith looked like with just 10 reference images:
From that, it only vaguely resembles the character; since we want our character model to, you know, actually look like the character in question, let's get more images in our dataset. I can't recall how many exact images I had in the Stan dataset, but I made sure to have a bare minimum of 30 images per character/folder upon trying again. Here is what Stan looked like afterward:
MUCH better! Anyway, after gathering enough images for your character (and again, I'd recommend at least 30 per character with various angles and positions), it's time to tag them. I won't go into too much detail here, as I covered this in the CivitAI's trainer: A simple, beginner's guide to training character LoRA using it and the Google Colab: A no-nonsense beginner's guide for training SDXL character LoRA! articles.
I do, however, recommend using a program like TagGUI to tag your images automatically. I like TagGUI because it not only automatically tags your images in a separate .txt file, but it also saves that file to your PC so that you don't have to manually download the training data after publishing your model to CivitAI. This also allows for quick reference into which keywords you should use to try to recreate a character/their appearance. Here's an example of Carol Olston, who I will be making an updated character model for shortly:
Again, the rest should be straightforward. What I do with TagGUI is to navigate to the outfit folders in question (again, let's use Lilith casual outfit (outfit), Lilith magus mode (outfit), Lilith red bikini swimsuit outfit (outfit), and Lilith school uniform (outfit) as an example) and manually type out the tag in question under the Add Tag field, highlight every image with Cntrl+A, smash that Enter key, and then rinse and repeat for each outfit tag. Then I navigate to the root folder of the character and add Lilith Asami (character) to every single image. After that, I click the Start Auto-Captioning button and let the program work its magic. After this, just repeat these same steps for all your characters until every outfit and appearance is prepped.
And that's basically it! From here on, just train your character like how I mentioned in the other training articles I mentioned and go from there.
The only other thing I can immediately think of adding is that, as I mentioned, with having a bare minimum of images per character, I would also recommend having a good amount of images per outfit. While at least 30 images will allow you to learn the character's appearance, if you're also trying to replicate an outfit, you will want an adequate amount of images to help the computer better learn what it should look like.