Billion Girl

Billion Girl (ビリオンガール) is a shōjo manga series created by Miwa Sakai.

The first volume has been published by Hakusensha and ran in Hana to Yume magazine. Later it ran in Asuka, and all volumes (including the first one) have been published by Kadokawa Shoten.

Story
Kanoko Ooba is a 16 year old innocent girl currently attending high school. Her family owns a traditional Japanese sweets shop since the Meiji period. When her family's company becomes bankrupt, a mysterious stranger offers to pay their family's debt, in exchange, he wishes to marry one of the daughters of the Ooba family. When Kanoko's two elder sisters refused to be married, Kanoko volunteered herself in hopes of saving her family.

Characters
She is the youngest daughter (a high school girl) of the old Japanese Confectionary shop "Ooba", which is on the verge of bankruptcy. She has 2 sisters who bully her around. She marries Houjo in exchange for her family debts to be paid. when she married taiga, she lived there for 2 months, and then they were divorced. Taiga's older bother, took her, and made her to be "tokiko" taiga little half sister. and so on. i cant be stuff typing this. go edit it yourself after reading it; it is really good.
 * Kanoko Ooba

He is the Houjo Family's 17th successor. He married Kanoko because of the 1 billion dollar loan. He gets lonely rather easily thus he has a playboy image. Also, his job is as a porn novelist whose works once, Kanoko was set up by her friends to read out loud during class.
 * Taiga Houjo

He is Kanoko's classmate. He is very handsome but speaks in a somewhat sissy way. He likes messing with Kazuma. It's pointed out that he has had a hard life when looking up Kanoko's family registery. He has a really caring personally and doesn't like any of the other girls in the class. He knows Taiga in some way.
 * Yasuhito Amamiya

He is the Houjo family's super efficient house keeper and is in charge of managing Taiga. He has a very helpful personality, doing everything he is told.
 * Obinata

Kanoko's and Yasuhito's classmate who is secretly in love with Kanoko, though it's not really a secret. In his 1st year all the girls in his class were to give him chocolates for valentine's day. The task of giving him the chocolates was given to Kanoko thus leading him to believe they were from her. She quickly told him they were not and he's been bitter to her since.
 * Kazuma Oshinari

A first year high school student. Wants to take over Ooba Confectionary shop when he gets older. A rather smart kid. Is a spy for Oshinari and Amamiya as the apprentice at the Ooba shop.
 * Tatsuya Mibu

Taiga's elder half-brother and half-cousin. They look alike (Taiga has long hair and Reo has short hair). His mother is Taiga's mother's older twin sister and it's because of this that he hates Taiga and his mother. He wants to steal Kanoko away for revenge,blaming Taiga for (indirectly, if at all) killing his sister, Tokiko, when she really drowned in a pond.
 * Reo Ijuuin

Volumes

 * 1) ISBN 4-04-924867-0 released on June 16 2001
 * 2) ISBN 4-04-924868-9 released on June 16 2001
 * 3) ISBN 4-04-924871-9 released on August 17 2001
 * 4) ISBN 4-04-924894-8 released on January 17 2002
 * 5) ISBN 4-04-924908-1 released on July 17 2002
 * 6) ISBN 4-04-924920-0 released on November 16 2002
 * 7) ISBN 4-04-924939-1 released on March 17 2003
 * 8) ISBN 4-04-924939-1 released on September 17 2003
 * 9) ISBN 4-04-924961-8 released on April 17 2004
 * 10) released on October 16 2004