Just saying that Hal wouldn't be hurt was as good as a promise to Liquid-- if he said that something would happen or that it wouldn't, he meant it. And when he gave his word on something, he damn well kept it.

"I promise."

Liquid hadn't even thought that he was capable of such a thing, but God help him, he liked Otacon. He didn't want to see the man get hurt any more than Heather did.