In anime, harem is a genre that deals with one male protagonist surrounded by a cast of mostly girls and potential romantic interests. This comes the Japanese word hareemu ハーレム, referring to the same genre, which itself comes from the Arabic word ḥarīm.
A harem which is the opposite: one girl surrounded by guys, is called a gyaku-hareemu 逆ハーレム, or "reverse-harem."
It's a well known fact in harem anime the protagonist doesn't matter. People watch this stuff because of the girls, not because of the guy.
So the guy often has a bland personality, he's a self-insert with no willpower or ambitions of his own, an yes-man that gets pushed around and can't take a stand for anything besides being a generic cavalier and saving damsels in distress because of some cliche reasoning.
Meanwhile, are designed following diverse tropes, such as tsundere, kuudere, yandere, they have ahoge and zettai ryouiki, some girls are older oneesan, some girls a pretty bishoujo, some girls are small lolis, some girls are actually guys, some girls are pure otome, some girls are rotten fujoshi, some girls are trash, some girls are masochistic do-M, some are delusional chuunibyou, nerdy otaku, stylish gyaru, assertive nikushokukei, etc. etc. etc.
It's all about the labels. You'll see that normally each girl fits one label and one label exactly: you don't get to have two tsundere in the same harem, that's redundant, the niches have to cover maximum surface to be liked by as much audience as possible.
Which is why most harems suck, by the way: the characters aren't introduced because the plot needs them, they're introduced because you don't have a girl that fits a specific niche so you make a girl that fits that niche and shove her into the story somehow.
And none of them have boyfriends for some reason. They're all attracted to this one single dude, who's a chick magnet despite being the most boring guy on the face of Earth: who's never ugly, but not that hot, who's not strong or muscular, who's not smart, who's not rich, who doesn't even have a hobby. Who's not anything. And they will fight and hate each other over that guy.
Since the plot is designed to be in shambles from the start, lots of harem anime don't even bother doing plot and just go the slice of life way. If they do plot, they may do it the over the top comedic way to save people from expecting much. The ones that try to be a harem with serious plot inevitably fail very hard.
Having the guy is obviously a problem. Since it implies relationships with the opposite gender will be going on, and for some reason that means fanservice, a lot of peeking into people taking baths, and sometimes accidentally seeing things that shouldn't be seen.
To solve this problem—and by "this problem" I'm talking about the protagonist, the guy, in case I'm not being clear enough here: to solve the problem that is the guy existing at all—some anime don't have the guy at all, choosing the much more pure and much more better CGDCT genre. Sumber https://japanese-with-anime.blogspot.com/
