Jeez dude, the first promise with a follow-up j/k was fine, but when she asks again, maybe put some thought into asking some clarifying questions before making such an open ended promise and end up agreeing to take on a whole country or something.
I get the feeling Hiro was getting involved regardless of the promise/offer he made. Why? Because legally, he's got her for 7 days. I don't think the villain will wait that long.
He wanted a pretty cat girl so much in the crew that he forgot every bit of the caution he has previously tried to maintain when dealing with people. He's not cautious at all when fighting, but he's pretty careful when approaching people or making promises.