I think the person who can belong to anyone's harem...
Airin? She's so sweet and cute and hentai,
but i can't think of her as the person with a harem.
Like she could go in lots of people's harems:
such as Yuko-san (Furukawa at least really respects/likes her...might be Kawa's oshimen. Plus, Yuko might have a harem with all her pairings. Furukawa might as well be that girl who gets the least screen time as a love interest in that story. Think of Negima if you've read it, read on, if not, ignore the rest of this parentheses or else beware of spoilers.
Mahou Sensei Yuko with her Mahora Gakuen kiddies... her favorite student, the lover Acchan (Asuna) and the midget Takamina vampire who has a crush on her (Eva) as well as her tsundere childhood friend who loves her (Anya) who would be NyanNyan... Furukawa can be Setsuna (at least the main chara and this one kiss~ :p) and the ojou-sama (Konoka) can be Rena [cough YukoRena fans, sorry, but if this was in the Negima verse, it'd be RenAirin dominating over YukoRena or KawaYuko]...that type of thing and the pairings are actually real in most cases. And ta-da, the droning on of Shino in this case is finished.)
or Rena (no need to really explain. RenAirin--you know how M-kawa is. Plus Rena = le girl with the harem in many cases)
Churi (well, rather than in the harem, more like the partner, but Churi is actually a mini-harem king too... ChuriNon, ChuriRena, ChuriJuri, Furuyanagi, ManaChuri...wow)