Kazimir represents Karou's past: sexy, reckless, fun, but full of regrets. See, right after Kazimir takes Karou's virginity, he breaks up with her. Akiva, on the other hand, is Karou's future: sexy, reckless, fun, but full of regrets (when he breaks her wishbone and kills her guardian, Brimstone). Oh wait... they don't seem that different after all, do they?
The novel tries make us think that Kaz and Akiva are totes diff, and that Akiva is, like, so much better than Kaz. The novel opens with Karou being all broken up about Kazimir:
How stupid she had been to fall for this petty, pretty street actor, dress up for him and give him memories like that? (1.29)
Karou even describes Kazimir as "Beautiful as heartbreak" (2.50), which is a wonderful metaphor, we think. Kazimir tempts her once more, but she denies him. She uses her wish magic to play pranks on him. When you think about it, their whole relationship dynamic sounds kind of fun in a romance-novel sort of way.
But Karou and Kazimir just aren't meant to be. And Karou and Akiva are, on account of the whole resurrection thing—Karou is Akiva's past love reborn. So Kazimir just gets constantly disparaged behind his back and cursed with an itchy rear, while Akiva lights Karou's fire.
So, yeah, Kaz isn't the coolest guy around, seeing as how he took Karou's virginity and then dumped her. But Akiva commits genocide. Who's got better boyfriend potential? You decide.