Often in normal dreaming I am aware of a presence or unseen DC either next to me or slightly behind my field of vision. When aware of it I assume it’s a benign dream companion of some kind but have never become lucid and engaged it. Is this common? What is it? Anyone ever engaged it?
Nah… I guess it’s your SC which is playing tricks to you. You should therefor take advantage of your situation - use that “person’s presence” as a RC, since it allways occurs in your ND’s.
You should set a goal (in case of an LD) to find out who that mystery person is ! That might be quite an interesting lucid adventure, who knows ? Personally I never experienced such a thing.
Remember, it’s only in your head. Everything that happenes is you, so nothing can harm you 
!
Have fun !
I certainly don’t feel threatened by it in any way. It feels benign. Maybe even friend like. For sure it’s in my head but it could be more than just a simple thought form. You make a good point…I will remember to use it as an RC and would definitely engage it when I become lucid. Cheers.