“What copyright protects is the work in the form in which it is fixed. That is, copyright protects the "work" as embodied in a physical representation, which is no doubt what he meant when he said copyright protects the physical representation.”
I confess (C) is a minor part of my IP practice. But I think you and the “information specialist” may have understated the case. Doesn’t (C) protect an original work of authorship - fixed in a tangible medium of expression - from which it can be perceived. “It” referring to the original work of authorship - not the tangible medium of expression from which it can be perceived.
Turn back the clock. I work-for-hire for Walt Disney. We want to come-up with a cute cartoon short. I get an idea for and draw pictures on transparencies (celluloid medium?, assuming such accepted by Register) of a cute little mouse - call him Mickey. (C) is applied for, submitting a transparency depicting my original art work on transparency(ies), and (C) is registered.
Disney’s cartoon short is a wild success. Somebody else decides a cartoon strip in the Sunday paper in which the cute mouse is the main character is a great idea, reproducing (copying) Mickey on newsprint, not on a transparency or pressboard or canvas, whatever "medium" was submitted to the Register. Differences in medium (newsprint vs. transparency etc.) notwithstanding, is there no potential (C) infringement (leave TM asside)? Derivative work? It think there is. It is the “work” of original authorship itself, not necessarily the tangible medium of expression from which it can be perceived, that controls.
Having typed that, I have an admittedly fuzzy recollection of a case from early 2000’s in which a manufacturer of sport shoes (Nike? Puma? Addidas?) registered (C) for a rendering (drawing) of a sport shoe, and later tried to sue a manufacturer of sport shoes for (C) infringement who was selling shoes that looked just like those in the rendering. They lost. Court said, if I recall, (C) did not extend to physical look-alikes bcs they were not “copies” of the rendering (drawing). But what about figures ("dolls") of action characters like Superman. Spiderman, etc., anything owned by DC or Marvel comics? If I start selling Spiderman figures w/o permission, I know I will catch flack. But under what theory?
Returning to OP, IMO your (C) registration for that "character" fixed on paper should provide protecton for the same character expressed in other media (e.g. ojn a computer screen). As for "expression" , not sure what you mean. Short "expressions" in sense of a slogan w/o content usu don't qualify, but the way an idea is expressed can be subject to (C).