Botan is Yusuke's assitant and they help each other out through tough times, like in one of the Yuu Yuu Hakusho movies. In the very end of the show, Keiko and Yusuke get married. Enough said about them.
Kuwabara
Kuwabara did think Botan was very attractive at the beginning of the show, then he met Yukina and he fell in love. Botan didn't even seem to mind, she called it 'a long distance relationship. How romantic!'
Kurama
...I have nothing against these two! However, Kurama did have a lover, who's name was Maya. There is proof of her exsistance, she's in the Two Shot Manga for YYH. From the picture shown, you can see Kurama and Maya did have feelings for each other. Sadly, after a demon attack placed her life in danger, Kurama had to erase her memory. Then he joined with Hiei in partners in crime. Thanks a lot to nadarelle! for being the first to bring up Maya. Although I do support Kurama and Botan, Kurama's somewhat broken heart probably belongs to Maya herself, even though she might not know it anymore.
Koenma
Let's be fair, Koenma and Botan aren't too bad. But I'm not giving up on Hiei. :D People say based on how long Koenma and Botan have been working together, they must know each other inside out. However, being around a person too long doesn't always end up with a happy ending. Being around each other for all enternity can have conflicts. Lets face it, Botan can be annoying. Koenma can be demanding. Clash that together and you have a problem. Yes, Botan has a duet with the Reikai prince, but she also sings with Yukina and Keiko. Does that mean she's a three-some with them? *pauses* Okay, that came out wrong. Erase the images in your mind! Let's get a better example. Hiei and Kurama sing songs together [around the campfire!], but it purely implys they are best buddies. Botan and Koenma's song may show how good of friends they are. After all, they like disgusing themselves in costumes so why not party together? Last reason and I promise no more hard core rivalry. Both of them may have mysterious pasts, but what do you expect from Koenma? I doubt him going off into a dangerous and daring journey.
Keiko
Wow, this is sort of werid. I haven't seen anything about them yet...I think? I have bad memory. Anywho, this is not likely to happen. Botan takes care of Keiko because they are friends, and because Yusuke could or might have made sure Keiko was okay while he was out on missions. Besides, I think Keiko and Botan think of each other of sisters. Botan comfort Keiko when she's worried about Yusuke and they look out for each other. They showed that when Keiko was being hunted down in her school. Not to mention in the last episode, Yusuke and Keiko well...kiss. Not only that, but they did get married, or Yusuke proposed to Keiko. All proof of Yusuke and Keiko's relationship could be found on Spirit Gun.