Context: Shinku 真紅 explains how proxy battle-based anime plots work to a main character who just got himself into a proxy battle-based anime plot via magic contract.
ii koto? いいこと？ (interjection.)
Just pretend it means "let me tell you something good," or "do you want to hear something good?" or "do you know what's good?" Or something like that, except it's not used before telling someone something good, it's just used to call their attention.
