How to change a character's direction in relation to the camera

Okay, so here's my issue. I've started making a game recently, and I'm still on the character controls. Now, I can move my character in four directions, forward, left, right, and back. He turns on each direction and I've also figured out how to move the camera around him. However, my issue is that when I change the camera direction with the mouse (using the Mouse Orbit script), my character doesn't turn with the mouse.

I would like to know how to do this in C#, because that is the language I'm making all of my scripts in. Help would be appreciated. Thank you.