Symbolism, Imagery, Allegory
Have you read up on coal and ash elsewhere in this section yet? Because you totally should. And no, not just because we break them down as symbols for you—because they represent pretty much the opposite of what hearts do. See, in Everneath, hearts are used to symbolically show humanity. (We're not talking about this kind of heart here.)
When Nikki first meets Mary at the soup kitchen, Mary spouts some nonsense about age and what year it is, but then she looks at Nikki "with clear, dry eyes" and says to her: "You broke a heart" (4.19). This seems to come out of nowhere, right? But later Nikki learns that Mary is actually Meredith, who started out Nikki's age but didn't quite make it through the Feed intact. In fact, she even went to school with Nikki. And so she knows that Nikki broke Jack's heart. And in saying so to her, Mary's offering a clue that she's not exactly as she seems.
While it's a bummer that Nikki broke Jack's heart by disappearing for the Feed, it also shows that Nikki and Jack shared a very human connection. And this connection is what enabled Nikki to stay young and sane during the ordeal of the Feed. In keeping each other in their hearts despite their separation, Nikki is spared some of the more gruesome side effects of being Fed on.
Cole, on the other hand, doesn't have a heart. At least, not one that's inside him. He seems to think that he shares something with Nikki based on eating her emotions, though. He tells her: "We shared a heart, Nik […] Your heart is in me now" (25.82). She promptly corrects him, though, saying, "Not my heart […] Just some of my emotions" (25.83). To which he replies: "Same thing. It belongs to me. And so I belong to you" (25.84). But are a heart and emotions the same thing? Is a heart just a collection of emotions? Nikki certainly doesn't seem to think so.
Again, then, we see the heart aligned with humans. Cole thinks he knows what's up with human hearts, but Nikki believes he's wrong. Perhaps the key difference lies in entitlement: Cole seems to think that he's entitled to Nikki's emotions—he says it "belongs" to him—but that's not how human relationships (are supposed to) work. Cole Feeds on Nikki's emotions, but she and Jack share themselves with each other.
Which leaves us with one question: What do you think is up with Cole's heart? We know he has some form of one—Nikki tries to destroy it in order to kill him, but fails. So is this a literal heart, the kind that thumps in our human chests? Or is it the more metaphorical kind, the heart we refer to when we talk about love? Depending on the answer, Cole is either technically still a little human—or emotionally so.