Why in the world are you still using a generic pronoun for general use, referring to Kou? Kou identifies as female, and the use of -kun is meant as a joke because Nao misgendered her originally, and it stuck. It's a running joke. As was said below, 'they/them/they're' should only be used in translation to English for people who honestly don't know how Kou identifies. But it's been made clear in the original, and is only needed in the translation to English because of how the cultural differences of the languages make use of personal references. This is legitimately an intentionally wrong use of pronouns.