This is such an interesting discussion, I really enjoy reading the thoughts from everyone who has contributed.

On the one side, there's the perspective that your dream guide is a manifestation of your subconscious, which knows more about you than you know yourself.

On the other side, there's the perspective that you're in complete control of your dreams, so believing in a dream guide is simply tricking yourself by limiting your own dream-capabilities.

It basically comes down to the standard theological discussion of "Can God microwave a burrito so hot that even he cannot eat it?" except ours is
"Can we create a dream-being so wise that even we cannot match it?"

I think the deciding factor is whether or not you've got full lucidity or not, and conclude that a dream character is a useful tool for achieving full lucidity, but it might be best to attempt to merge with them instead of letting them remain a separate entity, if you're working towards full lucidity.