Someone who understands when I need my own space to give it to me and not try and 'fix' me instead.
Fixing, really, not ever.
Someone I can vent to without judgement, and again, without them trying to fix me. Unless I specifically ask for their input.
Someone who can set really firm boundaries and if my behaviour or mood gets out of line, isn't afraid to point out that I probably need to find my own space to calm.
And someone who isn't afraid to tell me if they feel hurt or whatever their feelings on.
Basically someone who isn't a fixer and doesn't even consider walking on eggshells around me to not 'set me off'.
I think this changes for everyone though. When your sufferer is in a good mood, or fairly level headed, whatever their baseline is, it's probably a good idea to set boundaries and expectations for both parties and come up with some verbal cues that basically let each other know that you need to find your own space for a bit, instead of letting the situation escalate past the point of no return.
Oh, I also needed my SO to help ground me, before I learned how to do that myself.....
Right, so the biggest one: Communication.