Kennedy is a pain in the backside, and that's being relatively polite about her.
I despise Kennedy. In a fight between Caleb and Kennedy, I'm rooting for Caleb. I hate the way Kennedy pursues Willow and won't take no for an answer. Willow clearly wasn't ready to move on yet Kennedy still kept going at it until Willow eventually gave in. I hate the way Kennedy is suddenly included in Scooby meetings and put in charge of training the potentials, especially when she was a bully to Chloe who then committed suicide. Kennedy did nothing to earn a place amongst the scoobies, it was just handed to her because she's dating Willow.
I especially hate the way that when Willow looks like she's about to stand up for Buffy, Kennedy ROUNDS on Willow and says "Why are you always sticking up for her!" Willow hadn't even said anything at this point, she just stood up. The point is why wouldn't Willow stick up for Buffy, they've been friends for 7 years! Kennedy seems to think she can just come in and decide what Willow does.
I hate Kennedy. I wish she'd been one of the potentials to die, that would've been the highlight of the season for me.