Thank you. I didn't know what I was missing until your build-in shaders.

Hum, all this is pretty new to me, so it may be a stupid question, I'm sorry in advance.
But I successfully installed the tool in Unity, then imported my character (here called Cecile) in my Unity project, with all textures and materials, as well as the .json file I've found.
But still the tool seems to not recognize any CC3 character (I don't know if he's looking in the scene only or in the entire project, but even placing an instance of the character in the example scene doesn't fix it).
What am I doing wrong?

You don't appear to have a textures folder for that character.

Each character export should consist of:
character_name.fbx
character_name.json
character_name.fbm (folder with diffuse and normal textures)
textures/character_name/... (all other texture folders)

Without the textures theres nothing to setup for the character so the tool does not detect it as a valid importable character. There should be hundreds of textures in the textures/character_name/ folders.

Did you export the character directly into the project or copy it in from somewhere else?

Yeah indeed, all the textures are inside the .fbm folder, like that :
Does he need to have all single texture in here, or is it ok if there are some missing ones? Because I tend to delete those I don't know how to set up :x

The file structure as exported by CC3 needs to kept exactly intact, otherwise the tool will not be able to find the textures. And it uses all of them.

Got it ! I exported it again (ticking on the "embed texture"), then the auto setup tool recognized it and managed to make a prefab ^^
But the result is like that, the character got normal map and so on, but looks like all the base colors are missing, so it's all grey. It also thrown only one error ("failed to create folder"), but that's all.

Don't embed textures.
Never embed textures.

Oh ok, I didn't know it was a trap, haha.
Anyway, I finally made it ! :D Let me show you the results. Here is the character I have on CC3 :



Here is the character imported into Unity with Embed texture ticked on and some manual import of Diffuse textures here and there (still something missing on the eyes, but all the other parts are very nice) :



And here is the same import but without "embed texture" (looks much older, which is what I wanted) :



Long story short, thank you so much for this tool ! It finally saved my project, especially the hair, which was a total disaster, and which finally looks pretty nice now !
Of course the final result is not quite similar to what's in CC3, but for my scope, it's more than enough, so once again... Thanks a lot ! And keep up this wonderful work ! :D :D

The normals in this last image look very wrong, like the normal map textures are set to sRGB colour maps instead of Normal maps. Notice how the light reflects off the character in the last image completely differently to the one above? This suggests the normals maps are in the wrong colour space.

When you imported the character again without the embedded textures did you remember to run it through the High Quality material set-up again? (And if so did it finish without errors?) Because it looks like a lot of the textures haven't been set up properly.

Yeah indeed, I reduced the Normal strength inside Unity, and it looked a bit better.
And yes, using your tool, I always clicked on the High Quality button, and always redone the process when importing a new character.
Usually, when a Normal is not correctly imported, Unity spots it and suggest to fix it automatically. That's what he did yesterday.
And aside the error I told you yesterday ("failed to create a folder"), there's also this one:
"'file:' dependency [com.soupday.cc3_unity_tools] was not registered because its location is invalid (D:\Documents\Unity - Projet Varmalis\Assets\_A ranger\cc3_unity_tools_3D-0.1.9.3d). Make sure the package is not located under the Assets, Library or ProjectSettings directories."
But this is another thing, I don't think it's related to normals, i don't know.
What else could I show you? This is the Head material, and the Normal textures are correctly labelled as Normal in the inspector