Ansem studied the Heartless and wrote these notes about them. These are all the locations of the Ansem Reports along with the Ansem Report Text Included.

Ansem Report 4:

Receive from Aerith at the Hollow Bastion Library after saving Kairi.
The Heartless appear in groups and are multiplying rapidly. I’ve provided them with both living and non-living samples. They’ve responded only to the living. They seem to multiply after absorbing something from the living creatures. Their prey vanishes without a trace.

I believe the Heartless are taking hearts. They are born from those who’ve lost their hearts and thrive on hearts seized from others. The hearts taken by the Heartless become Heartless themselves.

Though I lack proof, I’m confident in this hypothesis. I must also study their behavioral principles. Though they lack emotions, they do seem to have some intelligence. How to communicate with them?

It just occurred to me: Could they be the darkness in people’s hearts?

This page contains: Kingdom Hearts Ansem Reports, Ansem Reports, Ansem, KH Report, KH1, Aerith / Aeris, save Kairi, Hollow Bastion KH1