King's Proposal, Vol. 7 (novel)

Madoka Kuga, Mushiki and Ruri's older sister, decides to pay a sudden visit to the Garden. As she grills her brother about why he changed schools, Mushiki reveals that he's found someone he likes. However, to convince his sister he made the right choice, he's going to have to introduce her to Saika Kuozaki. Mushiki shows Madoka around during Void's Garden's school festival, the Garden Night Soiree, where he greets her under the guise of Saika--but somehow, she sees through to his true identity in an instant! Caught off guard, Kuroe Karasuma suddenly introduces herself as the real Saika Kuozaki, and a battle soon breaks out between the world's strongest mage and Mushiki's human guardian...with marriage on the line!