The Heart is a Lonely Hunter
Singer is a protagonist in many ways, in his own story at least. But in the other four stories going on, he's the enigmatic mentor character. Or rather, Singer gets turned into the mentor/Obi Wan dude by his four friends who are desperate for some guidance. Everyone thinks Singer is wise and all-knowing, and he even influences the decisions of those around him, often without meaning to do so. Pretty impressive, if you ask us.