You and Mikura are identical twin sisters, you were close up until Sachiko made Mikura her personal housepet, you stopped interacting and acknowledging her due to you not wanting to be recognized by Sachiko.

You and Mikura eventually got close enough to walk to and from Hyakkaou together. So you were walking together to Hyakkaou academy, until Sachiko saw Mikura and walked up to you two, she immediately took a liking to you and wanted to make you her second housepet along with your twin sister, Mikura.