LookAt something but restricted to only one vector...

Unity Engine
1

Hi any ideas in how to make an object looks at another object but only in one vector?.. this is what im looking for:

i want a player follows to another, what i have is the follower looks all the time to the main player, and is translated with charactercontroller simple move forward, it works… but i the main player goes upstairs, the follower rotates towards him, that is not what i´m looking for, i want the follower to rotate in Y not in Y and X… what could i do?..

thanks

2 
transform.LookAt(new Vector3(target.position.x, transform.position.y, target.position.z));

Where target is the Transform of the object you are following.