Marmalade Boy

The storyline revolves around Miki Ishikawa, a high-school girl whose parents divorce only to switch partners with another couple and marry their new partners. The two newly formed couples then decide to live together in the same house, despite Miki's objections. The situation complicates when Miki meets and instantly falls in love with her new step-brother, the other couple's son. A love triangle soon develops between Miki, her step-brother Yuu, and her old flame/childhood pal Ginta. It becomes a love rectangle with the inclusion of Arimi, Yuu's ex-girlfriend. Meanwhile, Miki's best friend Meiko has a relationship with Na-chan, everyone's homeroom teacher. A similar love rectangle is set up involving Miwa (Yuu's friend, who pursues Meiko) and Ryoko (another teacher with a crush on Na-chan.) Relationships get more complicated as Ginta and Arimi fall in love and other characters (Suzu, Kei, Anju, Michael, and subsequently Jinny, Bill, Doris, and Brian) are introduced, resulting in what anime fans call "the love dodecahedron" because of its ridiculous complexity.