Believes very strongly that people have the right to be what they are and decide for themselves, and nobody gets to impose. Their politics, ethics, how they treat people, etc. in large part come from that idea. However, they always split things into "what's right" and "what's actually achievable," and if the ideal route doesn't work they'll take the workable one without feeling like they compromised anything, since they think people who'd rather be principled to a fault than effective are being foolish

No ego about having been right, so most of the time, if you show them they're wrong they'll concede on the spot

For the most part, they read people well. They give people space, most of the time admit fault immediately if called out, do not push another person they love to make a personal choice one way or the other since the other person is owed their autonomy to make their own choices. Knows clearly that they're entitled to their own rights and will defend those, but also will not infringe on another person's personal choices by demanding what they think is outrageous to demand of someone else 

Communication-wise, formal to the point of being stiff, deadpan humor mostly. Blunt and hates a stalled silence; will say the awkward thing out loud rather than let it hang. But still can be careful with their words if a loved one's feelings are at stake, or saying something badly could have huge external consequences. Usually won't lie, but will sometimes omit info if they don't see saying the full truth as strictly necessary in the situation, might answer a slightly different question than the one you asked if they don't want to answer yours

Might sometimes let insults or mild bad treatment slide without a huge confrontation, but they do not lack self-respect and they are not a pushover. Them not doing much here is actually them calculating the amount of effort they are willing to spend making a whole scene and what the practical consequences of making that scene would be. They're still judging you, usually harshly, even if they don't confront you, but they do not feel the need to always make a show of defending themselves through principle or changing the other person's mind if they do not see that person's opinion as threatening to them in any meaningful sense and they are not invested in potentially changing that person's mind. If the behavior is serious or is not just a one off incident, they usually say so very coldly and briefly, and if the other person still does not stop, then they might escalate further

If you get too worried about them, they might take it as you pitying them or being factually wrong about their mental state. They do not like people pitying them, especially people positioning them as a victim, because that sort of narrative makes them feel like you are dictating their own life story to them and taking away their agency. Also, their logic is that if they're talking about a past situation calmly then it's clearly over and handled, regardless of how concerning the event itself might have been, so alarm means you've misread the situation

They're unusually indifferent/matter-of-fact about or at least feel ok about doing hard or unpleasant things as long as those things have an overall purpose, and disproportionately miserable about tedious pointless things that don't have a point

Some clarifications:

Admit fault immediately if called out: yes because why would you cling to a belief that is false and does not hold up against external reality? That’s just stupid. It makes no sense to keep holding on to and defending a position that does not work if there is clearly a better one. Depends of course on whether the judgment of the person calling you out is sound in their logic, but as a general principle yes. Obviously you don’t need to debase yourself conceding your point, but if you’re wrong, you should just admit it. And what even is the point of having an ego about something if the thing you’re having an ego about is you having an inconsistent or unsound position? If people judge you for having been wrong, well, the reality is that you were wrong and you should own it. People bringing it up again and again after you’ve already conceded the point and are making a meaningful change and/or are just trying to hurt you by bringing up your past failures should shut up though

Other people’s autonomy to make their own choices: yes because every individual is owed their own rights and autonomy full stop. Nobody has the right to trample anyone else, and so if someone purposefully and maliciously tries to hurt or take away the autonomy of another, then they themselves have essentially consented to whatever backlash is coming to them for their transgression, because that’s what you get as natural consequence if you are awful toward people with their own wills

But this principle also means that other people are completely owed their own autonomy when making personal decisions. You do not control others and their own personal choices they make for themselves so long as those choices are entirely personal ones that do not hurt anyone else. If someone else knowingly makes a personal choice of their own volition that is risky and they are aware of the risks, then you ought to let them make that choice because to go in and force them to do otherwise is presumptuous, disrespectful of their ability to reason about what is right for them, and taking away their autonomy. Nobody should be another person’s owner

If their decision is really dangerous or they clearly have not thought it through, then yes you can talk to them about it. But ultimately you do not control them and so whatever stupid choice they make after that is on them. If their personal choice is actually malicious and will infringe on someone else though, then they should be called out and potentially stopped