Most character sheets are decorative. Three attractive poses, a colour swatch, a name. They look professional and they answer almost none of the questions that come up once the character has to appear in eighty panels drawn over six months, possibly by more than one person. A useful sheet is duller and considerably more valuable.
The test is simple: can somebody who has never met this character draw her in a new pose, from a new angle, wearing something she has not worn, and produce something the reader will recognise? If the answer is no, the sheet is a poster rather than a reference, whatever it cost to make.
Proportion in numbers
Write the head height, the shoulder width in heads, where the waist sits, how long the legs are relative to the torso. Numbers survive being copied; impressions do not. A sheet that says roughly seven and a half heads, shoulders two heads wide will hold a character together across a long project far better than a beautiful drawing will.
Include a plain front, side and back at the same scale, with horizontal guides. Uninspiring to make and constantly used. Most continuity drift happens because nobody had a straight front view to check against, and the character slowly became taller, or her eyes slowly migrated up her face over the course of a book.
Silhouette first
Put a black shape of the character on the sheet. If it is not identifiable as her, the design has a problem that no amount of detail will fix, since readers identify characters by outline at normal reading distance. This is the same test recommended for a whole cast in the notes on one base mesh.
Then check it against the rest of the cast as black shapes together. Two characters with the same outline will be confused constantly no matter how different their faces are, and the cheapest time to discover that is now, before either has been drawn in a scene.
Wardrobe and what she actually owns
A real person owns a limited set of clothes and wears them repeatedly. Give the character three or four outfits, drawn properly, rather than an endless closet. Repetition is realistic, it makes her recognisable, and it means the artist is not inventing a new garment under deadline pressure in panel forty.
Draw the hard parts: how the collar sits, what the back looks like, where the seams are, what happens when she raises her arms. Those are the views nobody puts on a sheet and everybody needs. The fun three-quarter hero pose is the one view you will never actually have to check.
Settling the adult read once
The character has to read as unmistakably adult from every angle, in every outfit, at thumbnail size. Settle that on the sheet with a documented head-to-body ratio, defined jaw and neck structure, and eye proportion that sits within the head. Then it is a property of the design rather than something each scene has to re-establish.
Do the crop test on the sheet itself: isolate the figure, remove all context, and check that the answer is obvious. Our standard on this is written into the reader agreement, and the drawing mechanics are covered in the piece on legible adult age.
Expression range deserves its own row on the page, and it deserves the difficult expressions rather than the easy ones. Happy, sad and angry are drawn correctly by almost anyone. Tired, embarrassed, guarded and quietly pleased are the ones that define a character, and they are the ones an artist under deadline will otherwise invent differently every time she appears.
Building a sheet that earns its place
Aim for a page that is boring to look at and impossible to work without: numbers, orthographic views, a black silhouette, three outfits with their awkward angles, expression range, and a short written note on posture and how she moves. Add a colour strip with actual values, not just hues, since value is what holds up at small size.
Then use it. A sheet that nobody opens is decoration, and the way to make it get opened is to put the annoying information on it, the things people would otherwise have to guess. The palette side of this is in the colour notes.