Kairi: If you can take her down quick, go ahead and fight her. But if you can't take her out before she goes off, or withstand enough absolutely brutal attacks to wear her out, don't even try it. She epitomizes the glass cannon style of fighting, and if she gets you on the back foot it's gonna hurt.
Shipping's all good though! She's a good choice, soft and sweet and stubborn. Might cause a few heart attacks when she gets reckless with her own safety though.
The Knight: Do not fight them. Do not even try. They are two feet of pure asskicking and if you somehow manage to kill them they'll just come back due to being made out of pure Void.
As for shipping with her, well, it's working out in Erku, sort of. She doesn't really feel romantic for anyone she's met there but honestly that's for the best.
Answer: Because you're fucking Kirei Kotomine