The Dress (ドレス) effect dresses Sabitsuki in a black mourning dress complete with black gloves and veil. Like many other effects, it is entirely cosmetic and does not give Sabitsuki any powers.
The Dress effect can be found in the Industrial Snow World by interacting with the dress on a stand which is in one of the buildings.
Go left in the Nexus, then go southeast to find the tiny house. Enter it and go down the stairs, ignore the elevator. You should move on to the next area and there should be doors. The first door you come across, you go through it to get to the Industrial Snow World. Going into that room, you will find another door which you simply go through.
Going straight up from the entrance, you'll find three cylinder shaped buildings with doors. The middle one will be where you'll see the dress on the stand!
- Pressing the action button makes Sabitsuki do a curtsey. When Sabitsuki curtseys near Cleaners, they bow to her.
- It seems odd that such mournful attire is only labeled "Dress". Perhaps this dress was commonly wore by Sabitsuki, another representation of her life being very difficult and taxing.