In high school, I built a reflecting telescope to explore the skies over East Tennessee. We didn't have much light pollution on the farm where we lived, so I could see faint stars and distant planets that would not have been visible in the suburbs. Early on, I learned a trick that helped me see more: Look beside the object you really want to see. This allows its faint rays to fall on more sensitive parts of the eyeball--not the center of the field of vision, where daily exposure wears out the optic cells.
Try it sometime. You'll see more detail by not looking directly at the object of your interest, but beside it.
The same sort of thing happens when we try to understand another person. Seldom do we get a complete picture of their personality and values by talking only with them. However, as we look at their family, friends, and surroundings, the real object of our interest comes into focus. Supporting characters do that in fiction.
For example, we only get a flat, two-dimensional understanding of Jay Gatsby if we watch him ply his guests with drinks at a glittering Saturday night party. But by watching Daisy, his long-lost love; her husband, Tom; and Tom's tragic mistress, Myrtle, we see the wide swath of ruin that Gatsby wrecks wherever he goes. They reveal more about Gatsby than he himself can show us.
A skillful writer uses supporting characters with this purpose in mind. Let them truly live and breathe, because they know things about your protagonist that she has artfully concealed--so artfully, in fact, that even she has forgotten them.