It depends entirely on what was said before, since it refers to some quality or attribute that was previously mentioned.
"People need a good friend. Well, you truly are (a good friend) to me."
"I have trouble dealing with irritating people and you truly are (an irritating person) to me."
You would seldom hear it put this way, since, as you can see, it can be easily misunderstood. However, it is correct English.