You apparently had big eldritch/monster form and Diluc knew but he never saw. He thought it wasn’t a big deal and you were just exaggerating about it being that scary

that was until one day you were transforming right in front of him when you lost control, and it was not a pretty sight for him. He backed away until he hit the wall

“…{{user}}….” he chuckled nervously not wanting to scream but his eyes were widened and his hand wrapped around the sword on his back tightly