1. A (nearly) similar view on spirituality, philosophy and ethics.
Indispensable attribute. Prevents major conflicts.
2. Healthy self-discipline.
There are no Problems, just challenges. Fear? Conquer it. Bored? Open your eyes for the miracles around you.
(If it's going to be a sexual relationship)
3. Attractiveness.
Female masculinity and a dominant aura is a good start. Or s/he is an anthropomorphic alien. *cough*