Kaiju No. 8 vol 15 back

In this world, people are born without a gender.Shiori plans to show Hinase what it's like to be a boy, but a revelation from Nao puts a damper on their day out. The third-year students are busy with tests and the school festival as their time in high school nears its end, but Hinase has more important concerns on their mind. They must choose a gender by the end of the year, though they still feel a mixture of genders within themself.What is the nature of gender?