In the center of a blank sheet of paper, write your main character’s name. Circling your main character, write all the people s/he will encounter. Draw a line from that person to your MC.
When you have written everyone, no matter how small the interaction, write a brief description of how your character relates to that person. Are they friends? Enemies? Lovers?
Is it a teacher? The gas station attendant? A sibling?
As you write these relationships, decide if they’re fully formed (or as fully formed as one run-in can develop). Do you need to work on this relationship? How can you flesh these out?
Relationships offer dimension, plot, emotion, life, depth etc. Make sure EACH one in your book is developed.