
The heart, I think, is probably Allen's Innocence. The reason why I think Allen's Innocence is the heart is because there is a lot of indication of it.
First and foremost, when Allen was brought to Hevlaska, she predicted that Allen will be the destroyer of time which is an indication to the Millennium Earl. Since Neah himself is also part of the Millennium Earl, this means that the prediction wasn't talking about Neah but Allen himself. And Allen needs the heart to defeat the Earl. That's a big indication there.
Secondly, Allen's Innocence was supposed to be destroy by Tikki but was recovered. If Allen's Innocence was the heart, it would explain why it was able to survive Tikki's attack when other Innocence wasn't able to since the heart is said to be stronger than regular innocence.
Thirdly, Allen's Anti-Akuma weapon is the invert of the Earls. A lot of people believe it's because of Neah but I beg the differ since Innocence are separated from Noah. Which means Neah's existence should have no affect on the form in which the Innocence takes.
And lastly, the Apocryphos. He's hell bent on capturing Allen to assimilate Allen's Innocence into him. If Allen's Innocence isn't the heart then why would he go through so much length to protect it? Other Innocence are being hunted down and destroy yet, he doesn't care.
If it was really Allen's mother who's influencing the Apocryphos then the Apocryphos would care more about Allen than the Innocence. Yet, the Apocryphos cares more about Allen's Innocence and could care less as to whether Allen lives or die. It wouldn't make sense as to why he's so obsess with Allen's Innocence if he's trying to save Allen.
I've been watching this and rereading this many times..... its just theory about the heart and i think its none of them in the present characters because the thought of allen's background they never talk about the real biological parents of allen it just began when allen met mana they never know where he came from he just came to mana at the age of 5? Or toddler age so its just a theory i thought of maybe the heart is the mother of allen since the aphycocphse? Said the heart is worried for allen its like that since i also remembered the story of tegami bachi same like that and the sense of demons and innocense (angels) there is the holy mother that it can also symbolizes as the heart.... just a theory (づ ̄ ³ ̄)づ i wanna share