Board Thread:Questions and Answers/@comment-27671186-20160208180243/@comment-25942629-20160208213318

The human part of Guren cares about Yu and his friends and family. The demon side couldn't care less.

It's possible that they hid Guren's demon side from Shinya. Guren would want to protect Shinya, and Guren's demon side would want to keep him as a valuable pawn. Shinya would do something the moment he realized Guren had become a demon.