If you're planning on making this an actual, functional swimsuit I don't know how crazy you'd want to go with the zippers because they may rust and stuff and I definatly wouldn't make the zipper functional because, depending on how tight you make it, you may spontaniously find your self falling out of the suit.
Here are some patterns I found (dunno how much sewing your're wanting to do, but I assumed you're planning on making it from scratch because cutting into an actual bathing suit will make it lose it's shape) and how I would Kairi-ify them.
(top picture) make it pink with, maybe a light pattern to it to make it more interesting, a small zipper between the bust were it gathers with either a very large pull tab or a charm (like a paopu fruit or something) hanging from it. White straps holding the suit up and maybe some black lines under the bust (could use that for underwire if you like that in bathing suits) to keep with her color scheme.
same basic deal here. If you can replace the ring in the middle of the bust with the sipper, this would just be a two piece version of ^.
this one you could possible add more zipper to (down the middle.) But the other ones are cuter and feel more like Kairi to me.
"It's my mother's gun, it's always loaded."
Aqua (Kingdom Hearts: Birth By Sleep)
Princess Katia- Lost Kingdoms/Rune (making the mock-ups)