Together Alone

(from ebookjapan):

Kaoru, a commoner, and Reiko, a daughter of a conglomerate, who attend Aisei Women's Academy, are bitter enemies. They are complete opposites in their upbringing and personalities, but when their parents remarry, they end up becoming sisters. Although they repel and hurt each other, they gradually come to understand each other and deepen their bond, but eventually, Reiko develops a love for Kaoru... Kaoru and Reiko embark on a solitary journey together as they face the absurdities of the world.... Riyoko Ikeda delicately depicts the emotional turmoil of adolescent girls in this much-talked-about work!

Note: Also contains the one-shot Gomennasai: Ryoko Hagiwara, an elementary school girl, decides to stand up for Yasue Yamada, an impoverished girl in her class. Little did she know the harsh reality awaiting her... A work that frankly depicts class divide and privilege, and how children emulate the practices and discrimination of adults.