Officially named『kokoro program』.
A program made for robots and androids so they "can acquire emotions". Through experience and learning,『kokoro』grows and gives those who are not humans a『heart』.
The more developed they are, the more human-like they are.
They are developed by experiencing different kind of experiences which include emotions.
Nobel is said to have a very well-developed kokoro more than Kite.
Early in the story, it is said that KNoCC's kokoro are not that well-developed yet but it develops as the story goes.
Einsatz has an under-developed kokoro and he himself thinks that it is unnecessary though it can be seen that his kokoro develops too throughout the stories.
STAND-ALONE stated that the more developed a kokoro is, the more their specs don't matter in them.