Sandor and grizel dynamic?
I feel like a lot of their relationship is defined by temperance. By caution, but also by confidence. They're both incredibly disciplined, and that included how they interact with each other. They care deeply for each other and have for years, and yet each scene we've gotten between them is a controlled, intentional interaction (the confession scene being the one exception). They expertly draw that professional line between work/their charges and their personal lives.
I suppose in that sense, we can't truly know their dynamic. Seeing them through Sophie, we have only ever seen their professional personas, so they may behave entirely differently around one another alone. Doubly so since they're adults dealing with children.
Based on what we do know, though, I enjoyment seems to be a major part of it all. That may seem like a given in many relationships, but that's not always the case for fiction; Grizel gets genuine delight out of teasing and playing with Sandor, and Sandor sees her enjoying herself and enjoys that, even if games aren't his forte. They push each other to be better, but there's a clear respect for each other's boundaries. I have no doubt if Sandor genuinely didn't want to wear the silver pants, Grizel wouldn't make him, for example.
We could also say they strengthen each other, both in terms of skills--Grizel teaching Sandor to be sneakier--and in terms of mental/emotional fortitude. They are someone for the other to think of and fight to return to, a grounding home away from home.
Their pairing is sweet and stable, understanding and reserved and enduring. Mature, I think is a good summary. They're two mature people enjoying each other's companionship, but they keep it mostly to themselves and don't let it interfere with their work.