Maybe it's just because I'm a fangirl, but I like to think the "Heart" is Miranda. My reasoning is that her Innocence is *so* unique in what it can do. Seriously, compare the Time Record to any other Innocence. All of the others are offensive weapons, while Miranda's is currently strictly defense. Not to mention when you think of the word "heart", you think of mercy, compassion, love, etc. Miranda's ability to defend goes along with that pretty well. Also, the "Heart" would probably want to be an Innocence that isn't put into immediate danger, and isn't completely obvious. While all Innocence are put in danger, Miranda's is usually defended by several others with offensive capabilities so she doesn't get killed and so they can continue to fight with no/less injuries. And then, you'd expect the "Heart" to be extremely kickass and strong, but Miranda's seems weak at first, which would make it less suspicious.

...or maybe my mind is just fabricating reasons for Miranda to deserve more screen/panel time =/