Dude built her whole legend in his imagination. They go to different schools, so he would know exactly nothing about her. But since she doesn't engage in conversations with total strangers on the train, she must be a cold ice princess that's hard to approach. Yet I could bet the dude himself doesn't chat every morning with unknown salarymen and office ladies on the train. Does that mean he's an ice prince, huh? But I guess it's not that peculiar to form such an imaginary profile for someone who repeatedly caught your attention. It would be intriguing.
Nevertheless, it's weird the MC's imaginary profile for the girl made it to the description of the series.