In my own, I think it's Allen Walker who has the "Heart" because not only is he exceptional but he became a point breaker in the age of 15. Lenalee is a different case cause komui said that any equip type can be crytal type/ parasitic. so to point it out, maybe his arm being a sword/equip type is so unnatural. so He is the Heart!!!
I really think that Allen contains the heart. At first I thought that Lenalee 또는 Lavi contained the 심장 but, Allen was the main character, it is 더 많이 sensible that he contains the heart.