Essek constantly gushing about his partner but pointedly not giving his name hits me so hard in the feels.
Two formative childhood experiences for me:
I was severely, mercilessly bullied as a child at every school I went to even if they're was no overlap of kids, and authority figures either ignored me or directly told me it was my fault. I was socially toxic. Any other kid who publicly associated with me was also targeted for harassment. I was best friends with a girl around the corner but because I was a couple years younger (in itself an invitation for bullying) and a parish, we could never let anyone know we were friends.
I've been told I should be upset at her for this, but it wasn't her fault. It was the other children who made it a fact that she would be harmed by publicly being my friend. She didn't make those rules, we were both just honest that it existed and there was nothing we could do to change that. The best we could do to survive was at least protect her. And that benefited me by actually having a friend.
So if we talked about each other it was"my friend." No names. No acknowledging we knew each other in public. No introductions to other friends. Keeping that divide up was necessary to survival. I had a couple friends on the same freak level as we and we were in fact targeted with additional harassment to get to the other person. It was a legitimate threat to live with. At some point I just stopped thinking it was ever necessary to reveal who my friends or family are unless it's both explicitly relevant and necessary.
I learned to use the internet in the late 1990s when anonymity was considered a best practice. Don't give out your age, sex, location, or other identifying information. You don't know who is on the other side of that screen or what they will do to you if they know. Sperate your online and offline worlds to protect yourself.
This helped reinforce experience one because clearly adults also acted like those kids and this just normal human behavior no one will ever put a stop to that you need to be on guard for at all times. Build in air gaps so if one of you is compromised it's harder for the perpetrator to get to other people you care about. Defending them through anonymity is a way of showing you love them.
Also since some family are searchable through have state government jobs that right-wing nut jobs chips target them for, I wanted to make sure they couldn't be connected to me as a queer trans disabled person active online. In case something I said led to them being targeted.
(This is correct advice, even though it flies in the face of modern online conventions. There are tons of malicious people on three internet who will target you and anyone you love if they decide to hurt you.)
By default, I refer to people by their relationship to me, not their name. My friend, my partner, my parent, my family, someone I know, etc. Often I avoid gendering them to make it even harder to identify them. I have to consciously consider if the person I'm talking to has any reason to know my associate's name. Blacklist everyone, then whitelist exceptions.
I do this even if both people know each other because the specific association feels dangerous. Better to be viewed as acquaintances than a meaningful relationship that changes how either of us could be viewed. It's not even really a judgement on thinking the person is untrustworthy, I just don't want to spend any extra energy thinking about it. It doesn't even feel relevant because my relationship to this person fellas like it conveys more information that actually matters.
Essek knows both he and Caleb are being targeted by powerful people who have shown they will target loved ones to get to them. Additionally, tensions between the Empire and Dynasty are still high and it could very easily compromise how their own sides view them if it's known that they're romantically entangled with someone from the other side. It could also blow each other's cover and make their meeting places more vulnerable to attack. Especially if their enemies know they could hit both of them at once.
It's genuinely dangerous for their connection to be known, so they don't name names. It's not even a matter of whether Bell's Hells would intentionally misuse that information, but what they also could just let slip to the wrong person. It's not really worth the risk when "my partner" is all the information they actually need to understand him.
My guess is that Essek said "Bren" is hiss partner because they already know a Bren sent them to Astrid. And since Caleb no longer uses the name Bren it would be much harder to connect them. It would have caused more questions, more prying, and more risk to give no name for his partner when directly pressed. So he gives a truthful but less dangerous answer. The anonymity is an act of love.