The main character of the story has to be an average guy; after all, there’s really nothing interesting or otherworldly about a really good looking guy having a lot of women in love with him.  Keitaro from Love Hina is the perfect example of a harem anime lead.  He’s not especially good looking, he’s not particularly suave in social situations, not super successful, and most importantly he’s pretty dense when it comes to women.  This is a hallmark of harem animes, that the main character usually has absolutely no idea that dozens of women are in love with him.  The male lead is usually a good natured person who helps those around him out and keeps an upbeat attitude no matter what the circumstances.

The ladies of the harem anime are the other important part of the equation and this is where the laws governing harem animes are the strictest.  If you want to make one, you MUST have some, if not all, of these character types:

• The Childhood Friend: This character has either grown up living in the same house as, or the house next-door to the main character.  She has always loved the main character and has always spent most of her life looking after him.  Naturally her chances of actually getting him in the end are pretty slim.
• The Sister: Like The Childhood Friend, this character has always been in love with the male lead and is never actually his blood relative.  This character is pretty interchangeable with The Childhood Friend except there’s usually more sexual flirtation as there’s always a scene where the “brother” walks in on her bathing, etc.  The hallmark of the sister character is jealousy at the other girls starting to gain the attention of her beloved oniichan.
• The Tomboy: This character can usually be found on the softball, volleyball, and swimming clubs.  Her exposure to the main character is pretty much the first time she has ever felt anything towards a boy and her lack of experience costs her in being able to confess her feelings for him.
• The Ruri-Type: Ruri Hoshino from Martian Successor Nadesico was such a wildly popular character that anime creators haven’t been able to resist carbon copying her into their own shows.  The Ruri-Type is easily spotted since she has the body of a 12 year old, but is usually older than the others thanks to having missed a few years of school due to illness; so she’s conveniently 18.  This is convenient for the reason that a lot of harem animes started life as ero-games, so the game makers need a way to skirt around obscenity laws.  Like Ruri from Nadesico, these characters are always soft spoken, yet quite snarky and are always quite intelligent.  Her hair, like Ruri’s, is always from the blue color palette.
• The Rich Girl: This character is even easier to spot than the Ruri-Type since she arrives to school in a limo, has large curls in her hair, and laughs into the back of her hand. She’s also usually surrounded by falling rose petals.  She’s never a serious contender for the main character’s love, but she serves to get in the way of the other characters and pad things out a bit.
• Big Breasted Big Sister: This character is more comic relief than anything else.  While she does naturally love the main character and usually ends up with his face buried in her chest, she is generally the older sister of the childhood friend and doesn’t want to get in the way of things.  Her existence is primarily for stretching the limits of breast bouncing and she offers sage advice now and then, which is remarkable since she is usually portrayed as being quite dippy.
• The Dimbo: This character is part dim-witted, part bimbo, but with a heart of gold.  She usually wins the love of the viewers (but not the main character) by saying things like “I know I’m not good at anything,” or “I know he wouldn’t want someone like me.”  She’s usually friends with the childhood friend and plays a relatively minor part in the show.
• The Tsundere: This character type is defined as being prickly and unapproachable, but quickly reverts to being cute when in the presence of the person she loves.  A good example of this would be Naru from Love Hina.  This character usually is the one who ends up getting the guy. *

The show ending can switch around about who ultimately gets the guy, but Japanese law does require that your anime contain at least four of the character types.

The minor characters of the show are the classmates, of which you have That Guy, who always says something to get slapped.  You also have the metrosexual who makes overtures to both the main character and the other girls in class. As an added bonus you normally get super perverted fat guy who is also there just for comic relief.

The other absolute requirement for a harem anime (and most animes in general) is that the parents are not in the house.  They’re either dead, or working in another country; either way they never play any part in the show.

* Thanks to Mathew Snyder of http://theillien.blogspot.com/ for pointing out my omission of this character type