There are two main characters in the first SF/detective novel I had published by a big mainstream publisher. Vail is physically striking—a sort of future goth—and I had no trouble describing her. Eddon, on the other hand, isn’t striking: regular features rather than handsome, gritty and nuggetty in a Russel Crowe sort of way. (So easy to picture Russell Crowe—but then try turning that picture into words!)
I didn’t realise till my editor pointed it out, that I’d given no physical description of Eddon for the first hundred pages. I’d shirked the task! I worked in a description in the re-write.
Readers need the passport details—and early on. Colour of hair, colour of eyes, shape of nose and jaw, build, style of clothes. Not very exciting, but such details are a peg to hang pictures on.
The fact is, there’s probably no area of a novel where it’s more up to readers to do the job for themselves. Everyone forms their own picture of how a character looks—and everyone’s picture is different from everyone else’s. If the novel gets made into a movie, how often do people say, no, that’s not how I imagined him/her at all?
Passport details also help to mark the characters as separate and distinct. Blond(e) hair versus brown versus black versus ginger versus … The only problem is that the possibilities are limited, you can soon run out!
Another passport detail is age. It matters to set the ages of you main characters very early on, and, for younger characters, to set their ages in actual years. Again, not very exciting, but a necessary peg.
Side-note. There’s a rule of thumb for children’s and YA writing—set the age(s) of your main character(s) at the top or slightly above the age range of your expected readership. If you’re aiming at 10-14 year olds, your main character should be about 14 or 15. Young readers don’t like reading about kids younger than themselves.