In Japan, the birth rate has declined to a point where the country's future is in jeopardy. So to help solve the problem, government official Hizuki has chosen a school to test a new, totally reasonable approach: each student will be implanted with nano explosives that will detonate if they don't pair off by summer's end! For Daichi, who'd rather be dating his 2-D girlfriend, and eight other students, the decree is too muchof a hassle and they refuse to participate, escalating the situation even further! Now, thanks to their defiance, Hizuki adds extra deadlines and challenges to ensure that the nine will somehow fall for each other, lest they become literal dust in the wind. But with an odd number of students, whose loveless life will become a mushroom cloud of despair?
Both series have a yandere girl as one of the main characters, but one series is generally serious (thus it ends up being creepy) while the other one is a comedy (so the yandere is played for laughs). If you read one and thought that you were looking for something different (something serious or a comedy) that also contains one of those yandere characters then the other one might work.
If you want to read a manga which has a female lead who is nearly as enthusiastic about her object of affection as Koharu, but who is cute rather than creepy, then Anne-chan Tarou might, be what you are looking for.
In both Koharu no Hibi and Choku! the protagonist finds himself a girlfriend who is a major nutcase. OK, thats not exactly true (I'm referring to the first part of previous sentence, the second one is very true) it's not like the protagonist in any of those series was searching for a girlfriend, it was the other way around. Kouharu no Hibi takes a more serious approach and can be scary and disturbing at times, on the other hand Choku! is a comedy. If you though that Kouharu no Hibi is too serious, you should try reading Choku!, on the other hand if you want to read something with a more serious approach to the loony girlfriend subject than Choku!, then Kouharu no Hibi might be what you are looking for.
Both manga feature a scary yandere character as one of mains. The difference is: one manga is mostly serious and the other one is 100% comedy. If you read one and you want to read something with a different approach to the yandere character, go try reading the other one.
A strange new transfer student joins Akira Tsubaki's class - her name is Mikoto Urabe and she has the habit of hiding her face behind her hair and falling asleep during class. One afternoon, when Akira goes to fetch his bag from the classroom, he finds Mikoto sleeping alone at her desk. She wakes up and makes her exit, but leaves behind some of her own drool on the desk. Without thinking, Akira tastes the drool... and is surprised to find it has a sweet, delicious flavour! A few days later, and Akira mysteriously comes down with a high fever; he's confused as to what is wrong with him until Mikoto visits him and tells him he's addicted to her drool and is suffering from withdrawal symptoms. From now on, Akira will need a regular top-up of Mikoto's saliva or else he'll fall ill - but maybe this isn't such a terrible thing, considering that Akira is falling for Mikoto...
Koharu no Hibi and Mysterious Girlfriend X both feature, umm... mysterious girlfriends!? So if you found the theme interesting in one of those two manga, you could give the other one a try. Just keep in mind that unlike Urabe (who is bizarre at best), Koharu can be pretty scary and disturbing...