Character design leans heavily on clothing. Silhouette, period, class, occupation, mood and personality are all normally carried by what somebody is wearing, and a great deal of design skill is really costume skill. Remove the clothing and every one of those jobs is suddenly unassigned, which is why undressed figures are harder to design than most people expect.
This is a practical problem rather than a philosophical one. A cast that is instantly distinguishable dressed can become a row of similar shapes undressed, and that is a legibility failure with real consequences for how a scene reads.
Silhouette without costume
Clothing supplies most of a figure's outline. Without it, silhouette comes from proportion, posture and hair, and those have to be designed deliberately rather than left to whatever the base construction produces. This is the cast-level test described in the character sheet piece, applied in the harder case.
The practical check is to render the cast as black shapes undressed and see whether they are still identifiable. Most casts fail this the first time, and the fix is structural variation rather than surface detail, which is the argument in the piece on the default body.
Character carried by posture
How somebody stands says a great deal, and it becomes the dominant channel once wardrobe is gone. A character who holds herself carefully, one who takes up space, one who is constantly slightly braced: these read clearly and they are properties of the design rather than of any individual drawing.
Documenting posture on the character sheet, in words as well as drawings, is what makes it survive across a long project and across more than one artist. It is one of the most useful lines on a sheet and one of the least commonly present.
Marks and history
Scars, moles, tan lines, stretch marks and old injuries individualise a figure and give her a history, which is the particularity argument from the piece on the nude and the naked. They also solve the legibility problem, since a distinctive mark identifies a character instantly.
They should be documented and consistent, or they become continuity errors. A mark that appears and disappears across a book is worse than no mark at all, since readers who noticed it once will notice it missing.
Hair does more work than usual
With clothing gone, hair becomes the largest single element of silhouette and the most flexible design variable. Length, mass, whether it is tied back, and how it behaves in different positions all carry character, and hair is the one thing that can change between scenes without becoming a continuity problem.
It is also a rendering opportunity, since hair masses catch light distinctly and give a large simple figure somewhere to put visual interest, as described in the notes on cloth and hair.
Props and partial dress
A single retained item, jewellery, a watch, one garment half removed, does an enormous amount of design work for its size. It restores a little of the silhouette, carries period and character, and gives the composition an edge or a shape to work with.
Partial dress also tends to be more interesting compositionally than either extreme, since it creates contrast in texture and value across the figure. That is a design observation rather than a claim about content, and it holds regardless of what a scene is doing.
Planning the cast for both states
The practical conclusion is that a cast should be designed to be legible in both states, and tested in both. Doing this at the design stage costs an hour; discovering it in chapter four costs a redesign that has to be applied retroactively to everything already drawn.
It also improves the dressed designs, since a cast differentiated structurally rather than by costume is stronger in every situation.
Designing a figure that works undressed
Vary proportion and posture structurally, document both in words, give each character a small number of consistent marks, use hair as the main silhouette variable, and consider a retained item. Then test the cast as black shapes in both states.
Characters here are fictional and unmistakably adult in every state, as set out in the reader agreement.