Jack couldn't hold on to it for long. While Kairi may seem to be light, is there any proof? She's a Princess of the Heart, something very strange. Nomura has made it clear that we know nothing of the Hearts and it has many dark secrets.
To my knowledge, while Kairi may seem like she walks the path of light, what exactly is the path of Present? For all we know it could be anything. It could be Light, Darkness and Twilight... It could be the relm that unites everything, as that's what the Present does.
For know there is no proof that Kairi walks the path of light, what the relm of the present really is or that she isn't or is a Keyblade Master. The fact that she used her own special Keyblade, and used it semi-well and without it vanishing, hints that she just might be.