Hikaru and Kaoru are two girls who, despite being younger than Hazuki, are pledged by their parents to marry Seiji and Kouhei respectively when they grow up. While Hikaru is engaged to Seiji, Kaoru is engaged to Kouhei. Kaoru is more quiet and shy and can be a crybaby at times. But, despite her shyness, she can also be quite mean to Hazuki, but this might be possibly that she is jealous of Hazuki due to the fact Hazuki lives with Kouhei.

from wikipedia