i think naruko/mom loves him. Not for his heart but himself whole as a person. Naruko was eating their hearts but never manuel's since his was gone im guessing. It was kind of confusing but i loved every second of this manga.
I think that Naruko helped Manuel recover when he was in the hospital, by giving Manuel's heart to God. And ended up falling in love with Manuel after he took him in. Hence "he has no heart" and "he belongs to me".
Especially when naruko said "even if i gave just his heart back to god, the other parts are all mine". What's that supposed to mean???