Yuko Kosaka is the Suoh family's official lawyer. She displays very meticulous habits, such as wearing a face mask and gloves everywhere she goes to avoid illness, and ranting when her assistant buys her the wrong brand of fish sticks.
Kosaka grew up next door to Haruhi Fujioka's dad, Ryoji "Ranka" Fujioka, who was several years older than herself and her first crush even though she assumed, as many did, that he was strictly homosexual. She attended a private junior high and her mom told her to work hard so as to avoid ending up like Ryouji, a dropout. She also attended the same law school as Kotoko Fujioka, whose name was still well-known at the school even when Yuko attended years later. During her college years, Kosaka's family fell apart because of her father's debts. Kosaka had to claw her way into her esteemed position as a Suoh lawyer and, unlike Kotoko, is more concerned with money than morality. She is dismayed to find that Ryoji grew up to be prettier than her. To this day, he calls her by the nickname,"Yu."
Kosaka reports directly to Yuzuru Suoh. She stalks Tamaki Suoh for a time and has secret meetings with his grandmother, Shizue Suoh, to report on him. She convinces her to allow him to live in the main mansion in the hope that she will receive a monetary reward from Yuzuru. She visits Ryoji several times in an attempt to sever Haruhi's friendship with Tamaki to please Shizue, suggesting that Haruhi does not fit in at Ouran Academy and even arranges for her to study abroad in America for one year. Kyoya Ootori thinks she is suspicious and has his bodyguards keep an eye on her.
When Kosaka first meets Haruhi, she tells her that book-smarts are not enough to make it as a lawyer and that she will need to learn more about life and people if she wants to help them like her mother did. This also makes Haruhi realize that the reason she loves Tamaki is because he has greatly expanded her worldview and she wants to become a better person for him.
When Kosaka helps rescue Haruhi from kidnappers, Haruhi is further inspired and checks out a stack of books from the library about career women. She admires Kosaka and even seeks out her personal advice in Chapter 81. At one point, the twins note that Haruhi is easily swayed by "professional-type" women, like Kosaka and Nanako.