Shounen A

Shounen A is thought to be about 20 years old (though no one knows for certain), and is a frequent customer of Cafe Lindbergh, so much so that he is considered their shop "mascot" and has a special reserved table where he reads and writes. Even though he is familiar with the staff, he has never told them his name, using instead the psuedonym "Shounen A" for unknown reasons.

Spoiler

Real name: Shimazaki Haruna (島崎榛名).