The harem genre is quite prolific in today's anime world. Described as a series with a plot driven by multiple characters being romantically interested in one person, harems are a common genre to attach to anime and manga. It could be the main focus, like in Nisekoi or Quintessential Quintuplets, or one aspect of a bigger story, like in How Not to Summon a Demon Lord or many other isekai series. To the surprise of many, the harem genre's origin can be traced back to one woman.
Rumiko Takahashi is widely accepted as the harem genre's creator with her series Urusei Yatsura, which began publishing in 1978. She continued to play with this creation after Urusei Yatsura's end with one of her best known series, Ranma 1/2. First publishing in 1987 with the anime adaptation airing in 1989, Ranma 1/2 tells the story of a boy who has been cursed to turn into a girl when he's splashed with cold water. Only hot water will make him a boy again.
Because Ranma can switch between the sexes at any given time, he has a unique setup to his harem. In fact, he's actually the center of two -- one for each of his presentations. While modern anime have mixed it up by having one or two people of the same sex in a character's harem, no series has made a love polygon quite as complicated and messy as Ranma's.
Plenty of harem anime get complicated enough to need a relationship chart, but finding one with the same level of chaos seen in the official Ranma 1/2 art book is a difficult task. Ranma often has some relationship with all members of his harem regardless of what body he's in, but how he looks greatly affects their treatment of him. One of the most notable examples of this is Shampoo, a girl who has very different feelings toward him depending on what body he's in. To her, girl Ranma is the person who stole first prize in a tournament from her and she vows to kill in revenge. Boy Ranma, by contrast, is the guy who defeated her in combat and, by her society's laws, that makes him her fiance.
Like Shampoo, if a character is in love with one version of Ranma, they often have a hatred for his other half. Kuno, for instance, fell for Ranma's female side at first sight. However, he hates boy Ranma because he's engaged to Kuno's other love, Akane. In this way, the harem becomes more than just infighting between members. While the girls in particular do compete for Ranma's affection, many of them are also in conflict with the boy himself. Whether it's him seeing them as a friend more than a potential partner or his curse keeping him from being ideal, there are more issues than a typical harem setup.
Another interesting aspect of Ranma 1/2's harems is how one is rather stagnant while the other is like a revolving door. As a boy, Ranma has several women who, for one reason or another, are his fiancee. Either because of his own actions (like in Shampoo's case) or because of a deal his father made years ago and forgot about, Ranma has no less than four women who hold the title of fiancee. Others do make appearances, but they don't last for more than an episode or two.
While boy Ranma has many long-term suitors who become recurring characters, girl Ranma doesn't. The only constant is Kuno, who is just as in love with his mysterious "pig-tailed girl" as he is with Akane. Besides him, many boys take interest in Ranma's feminine form but often vanish from the story after a few episodes. Be it figure skater Mikado stealing his first kiss or having a brief engagement to Sentaro of the Martial Arts Tea Ceremony line, the boys stick around for small arcs and may return once or twice, but never quite become recurring characters.
Many have debated whether Ranma 1/2 still holds up to a modern anime fandom, but its place in history is sealed regardless of its relevance today. It helped establish the harem genre that anime fans know today, but it's also taken the idea to an extreme that has never been replicated.