IANAL but just to complete a minor lesson in authorship and copyright in the US. By default copyright is assigned to the original author of something...except is the work is done "for hire" in which case the hiring individual or company will own the copyright independent of whomever wrote the work. At any point the owner of a copyright (which is really a set of rights they are allowed) can sell some or all of those rights (or otherwise license the work) on an exclusive or non-exclusive basis. If the copyright is sold wholesale than the new copyright owner would typically change the copyright notices to indicate that. To further complicate things with game franchises such as this there are additional trademark issues and rights issues surrounding the character, spin-off (derivative) works, etc. which may or may not have been included in the sale which is why the binary files (presumably containing the art) may not be licensed under the GPL.
Bottom line is the IP rights are a messy thing.