What title or term is appropriate for a genuine or sincere person?
You might have to turn to metaphor. "Genuine, sincere, and honest": someone who doesn't hide or lie about their true feelings. "An open book" is a common idiom. A "plumb line" is something that can be relied on for accuracy. If the point is that your husband can trust you to tell him the truth about himself without flattery or prevarication, some poetic turn of phrase like "your unclouded mirror" might be appropriate.