How did Gyomei become blind?

Gyomei is blind, thus explaining the reason for the lack of pupils in his eyes. He was blinded due to a fever he had when he was a child.

How does Gyomei see?

Despite being blind, Gyomei gained the ability to see without using his eyes. Through naturally sensing the people and objects around him, Gyomei can create images of his enemies in his mind. Demon Slayer Mark: During his battle against the Upper Moon One, Kokushibo, Gyomei awakened his Demon Slayer Mark.