Two of my cats are red tabbies who look identical if you don't know them. It often takes a guest three or four visits before they realize we have two of them and they aren't the same cat, even though they're as different as night and day personality wise. What does this have to do with characters? You can learn a lot about creating characters from my cats.
At first glance the cats look the same. Red fur, long and skinny, short hair, similar weight. This is what first-time guests remember after they leave my house. "They have cats." They might remember how many (minus one) and they might remember the colors.
Occasional visitors realize that Darwin is the older and bigger cat. His eyes are yellow with white fur around the rims, and his face is square. Puck is the baby of the group. He's smaller when you see them side by side. His eyes are red, no white rims, and his face is long and pointed. He has some Siamese in his genes somewhere.
These are the characters you want readers to remember a little, but they really aren't important to the overall story. They have names, and serve a purpose in a particular scene, and you have to describe them a little to keep the action straight. But once they're gone, they probably won't be back. You give them just enough to tell them apart from the other people in the scene. They're there more more color and style and world building than substance.
Regular visitors find out fast that Darwin is a chatterbox, while Puck rarely says a word. Darwin often walks around with his tailed curled in a curly cue, Puck holds his almost parallel to his back, and the vet thought he'd broken it the first time he saw him as a kitten. Darwin's favorite toy is a felt ribbon on a stick, Puck's is a stuffed fox that is missing most of it's fur and is torn in several places. Both cats will bring the toys to you when they get bored and want to play. Darwin will meow while he drags his stick around the house looking for you. Puck will just drop it in your lap or fling it at your head.
These are your secondary characters and the ones seen most often hanging around your main characters. Readers know what they look like, their quirks, some unique details (often called tags) to distinguish them from the other characters. They remember these guys even after they read the book. They know some of their history, but nothing in depth. Just enough to know why they're helping/not helping the protagonist and to get a feel for their motivations.
Those who live in my house can tell you both cats were rescues. Darwin is a mama's boy and will curl up in my lap any chance he gets. Puck loves my husband best and hangs around him all the time. Darwin doesn't like to be petted unless he's in the mood, Puck can be carried around the house with no trouble. Darwin is agile and a great jumper. Puck has trouble jumping and studies each jump carefully. Darwin steps so lightly you barely know he's there, even when he jumps on the bed. Puck bounds instead of walks and you'd swear he weighed three times what he does. Darwin will run to me for protection when the other cats are pestering him. Puck makes a beeline for the patio when he hears the back door open.
This is your main character (or characters). Readers know all about them. What they look like, how they feel about things, their quirks, their skills, their flaws. They can anticipate their actions and worry about what they'll do in any given situation. They love them unconditionally and remember them long after the book is through. These characters have contradictions and are often their own worst enemies to getting what they want. We can tell who they are just by reading their dialog or internalization, even if no name is mentioned.
When a reader picks up your book, it's like having people over for a visit. How you want those characters to be remembered can give you insight on how much you need to reveal about them.
Which characters in your book are drop-by-guests, and which are friends for life?