Muurei

Muurei is the femslash ship between Muu Kusonaki and Rei from the Milgram fandom.

Canon
Although the specifics of their relationship is up to speculation, we do know that Muu and Rei are schoolmates.

In the Muu's first trial music video "After Pain", Rei sees Muu laying on the bathroom floor with toilet paper and water around her. Rei looks down at her with an unbothered expression, then walks past her. After Rei walks past her, the lyrics sing "I love YOU".

Right after that scene, we see things in Muu's perspective and she is running towards Rei. Muu reaches out to Rei while the lyrics sing "I love YOU" (again). Muu grabs Rei's arm, and she's sobbing while trying to apologize or say something to Rei. Rei stares at Muu, once again, with an unbothered expression. Rei snatches her arm away from Muu's grip but Muu immediately stabs Rei.