Not Interested

"How about a rematch sometime? Fair and square, no dark powers involved." asked Sora.

"I think I'll pass" says Cloud, brushing his hair back.

Edit: It strikes me that mentioning this is said in Kingdom Hearts would be useful information.

This...is not Cloud. It's a very polite, friendly way to pass on a fight. As originally written, it better mirrored Cloud's style from Final Fantasy VII. What he actually said was:

"興味ないね"

Which means, simply, "Not interested" This is the option you're given almost any time Cloud can refuse something in FFVII. It's a rather abrupt way to refuse, and the phrase very adequately sums up Cloud's indifference to the rest of the world. It should also be accompanied by a non-committal shrug, for maximum effectiveness.