Please note that I am not a lawyer,
but I played one in a High School play.
There is a lot incomplete information here with the discussion of protecting ones images. First off, let's just talk about how we got to any of the characters. These are all fan created works, I don't think anyone on this thread actually got the exact recipe from L&P. Reverse engineering does not automatically mean "infringing".
Now we move on to actual protections under law. Under 17 U.S.C. § 501, an artist who copies a copyrighted artwork commits copyright infringement because the artwork they copied qualifies as a “pictorial” work of art under §102. However, it is important to know whether the designs that the artist copies are registered with the U.S. Copyright Office. Yet, while registration is voluntary, and copyright exists from the moment the work is created, the design does need to be registered to bring a lawsuit for infringement of work.
More specifically, to bring any legal action to enforce the copyright, you need a copyright application with the US Copyright Office—there is no common law right to sue for copyright infringement. If the image is registered, then its owner also has rights to “derivative work” under §101 and §106, so the artist who copied the image does not have the right to create and sell their derivative work.
But, no protection is available for unregistered designs. While the UK has design rights akin to copyright, design rights in the US are related to patents and must be registered in a design patent application at the USPTO. Unless the image that the artist is copying is registered with the US Copyright Office, the other artist cannot seek a court order restricting its use. Remedies such as injunctions or damages would only be available if the design is registered.
Most people just know the first part of this, so they assume that every derivative work is infringing, in some manner or another. While that notion will always keep you safe from legal action, it actually isn't completely correct. L&P would have to register his images, and that means every one in his VN, which has the character he wants to protect, or at the very least the actual 3D model used, with the US Patent and Trademark Office. I would bet that was never done. So while you may find that
Phoenya selling images on his Patreon of his interpetation of Sophia might not be to your liking, I am not sure if it is technically illegal. But I am also not sure it is a fight worth having if L&P reports the "infringement" to Patreon itself.
Then we move on to the topic of "Fair Use", but that is a whole other can of worms.