This story related.
This will unlock after finding the Phoenix Mirror in the Hazuki Basement.
To access to the basement, go in the dojo. Use the mysterious key on the chest (The Mysterious Key can be found in Iwao's room at the Hazuki Residence). In the chest, there is a sword, use that sword in the hole that can be found behind the right poster on the wall.
In the basement, you'll need two thing to navigate : the flashlight (can be found under the phone in Hazuki residence) and a bulb (you can buy one at Tomato Store in Dobuita).