did you replace luigi's model with homers, or did you import a new model?
i'm not sure if this would cause the issue, but importing is the way to go.
also, did you use
these settings when importing?
the UV mapping is correct, just the way its presented in BrawlBox is different than what is on the left.
importing the original shaders is quite easy, all you have to do is open up the original lg.brres, open up the model, and then open up the "Shaders" folder. I haven't done this in a while so i'm not sure on the specifics, but whatever file is in there, you should export and then import into your lg.brres with homer imported, under homer's model of course.
importing materials is tricker (in my opinion), and its what gave me the biggest issues
check under homers materials. there should be two or three sub files, one should be for the face, and one should be for the body. there could also be one for the mouth, but that varies from model to model.
inside those there should be the main texture, and then Im_1 and Im_0. I highly doubt your model has those already in it because its a mario kart thing. you'll have to delete the textures with those names from the textures folder before you create new files in your materials that correspond with them. after you do that, then you can re-import those textures. Also, the names on the materials matter because they have to be the same as the name of the textures, or else brawlbox wont be able to find any texture to load into the material.
this is really hard to explain, and I don't have all of the time that I used to to explain this, but I hope this helps. using video resources and atlas tutorial is all i needed to figure it out for myself though. Basically, whatever you do, make sure the end result mirrors the original model. What I like to do is have a split screen with one side being the original model and one side being the model that I'm working on. you have to make sure that the model and all of its contents can be read and loaded by the game, so check back and forth between those two files to make sure everything lines up. If all of the materials in luigi's file have Im_1 and Im_0 in them, make sure your model does too. If luigi's model has Im_1 and Im_0 set to "true" under the face material, make sure your model does too. basically all the little stuff like that matters.
I hope this helps in some way, I know it was vague but you will have to learn some stuff on your own through trial and error, because no tutorial I have found tells you
everything.
one last thing is: that i'm not sure why atlas has importing the model and doing all of this before its even rigged. I personally find it pointless and redundant because you just have to do it all over again at the end. I'm not really good at making models or anything though so this isn't some professional opinion, but for what it's worth, I skipped over this step and just did it at the end, and everything worked out fine.
so i'm pretty sure both Blender's dae and fbx exporters aren't that good lol