The Mimosa Confessions Novel Volume 4 was written by Mei Hachimoku and illustrated by KUKKA.

“Promise me you'll always be a good older brother to her, okay?” Misao Tsukinoki clung to these words from her late mother, looking to her older sibling as a guiding light. But when she came home that fateful day to find Ushio wearing her school uniform, her respect and adoration instantly turned to scorn and disbelief. Why was Ushio suddenly dressing like this? When did the changes start? Or had the person she thought of as a brother been someone different all along?

Desperate and in need of answers, Misao retraces their life together from the present to the distant past: their father's second marriage, their history with Sakuma Kamiki, their mother's death—and a promise they made to each other long, long ago. Old secrets and buried truths will finally come to light as Misao grows closer to understanding Ushio's struggles. And with them, a tragic tale of a fractured family unravels in reverse.