Love Interest...

[King-Killer]

Everyone is in duo except Quistis. (and Seifer in the end)
Squall/Rinoa
Irvine/Selphie
Zell/Library Girl
Laguna/Ellone
Kiros/Ward
Seifer/Edea
Raijin/Fujin
Biggs/Wedge
Zone/Wattz
Xu/Nida
Cid/Edea <--- Call her Matron if you must, when she ain't evil.

Even Them:
Raine/Laguna <--- When she was alive
Julia/LagunaORCaraway <--- Ether way...
Ultimecia/Griever <--- She is junction unto him...(Don't quote this)
Adel/Rinoa <--- (Don't quote either)
Omega/Ultima <--- if you call them a duo of weapons (and don't quote this)

With that said:
Everyone is in a Duo except Quistis..
Although, she does have a fan club, the Trepie Groupie.
Maybe she can Hook up with Seifer in the end.
I'd love to see thoses two together.