Great question, kitten. I'd say a relationship in which both parties have a strong curiosity to deeply understand, connect with, and care for the other. And both show a stronger interest in knowing where the other is coming from through their struggles than their interest in defensing themselves, being oppositional, controlling, and so on. But also with both partners being open and vulnerable with each other about who they are and what they feel regularly, to a deep degree.
Add to this that there needs to be a mutual willingness to allow the other to be himself and herself--to be separate beings and let each other evolve however their spirits naturally do.
And, in romantic relationships, of course... the sexual desire needs to be present... not expected at the intensity it was during the courting phase when there were all those additional hormones flaming up and all. But still some occasional fire.
This was a good thing for me to be contemplating tonight. Thank you for asking.