"bren aldric ermendrud" is a separate character from caleb widogast. he's a young boy learning how to make magic. he's a deeply traumatized and indoctrinated teenager. he's the boy who curled up with astrid and eadwulf in a freezing tower for warmth all night.

essek never met bren. he met caleb and he's never known him as anyone else. if i'm remembering correctly, caleb never even said the name "bren" to him during the campaign, and neither did any of the nein.

essek knows caleb widogast. he knows the man who held up the object of his worst crime and then kissed him in the bowels of a ship and made a floor of infinite stars for them to walk through together. he knows the person who healed over bren's wounds—thinly, but enough. he knows the man that the boy has become.

astrid knows bren. she barely knows who caleb is. she still calls him bren after hearing him referred to as caleb repeatedly. she can't know him as the man he is, she only knows the boy. there's some of bren in caleb, but there is no caleb in bren.

essek saying "bren sends his regards" is him gauging astrid's reaction, on one level—if she freaks out, which she did, she's in opposition to caleb's cause and thus a threat. on another level, it's essek delivering a very different subtextual message from caleb: "the boy who loved you is giving you one final warning."

because essek is a threat to astrid. their last meaningful interaction was slinging spells at each other in the blooming grove. and that's funny in a "current boyfriend vs ex girlfriend exclusively fight each other" kind of way, but it's also deeply tied to caleb's recurring theme of transformation. "bren sends his regards" also means "i have healed enough to love enough for someone else to know this name and use it with my consent. and this someone else is your enemy. what does that make you think i've become?"

it also does a fantastic job of communicating subtle offscreen discussions that have happened over the years since the end of c2. we don't have the details of when or how caleb told essek his birth name, but we know that he did, and we know from all of c2 that the name bren occupies a place of immense emotional weight for caleb. it functions similarly to a deadname in terms of who uses it and for what purpose. trent exclusively calls caleb bren to wound him and place himself in a position of power. astrid calls him bren to remind both him and herself of who they used to be—same with eadwulf, though obviously he's not here.

the first time we hear essek say "bren" is on caleb's behalf and confronting one of the narrative representations of caleb's trauma. it's four words that manage to communicate "i, your enemy on a hundred levels, can speak for both the man i love and the boy who loved you, because i know him in his entirety."

astrid knows bren and essek knows caleb, but only essek can speak for both. because at some point, caleb gave bren to essek. and we know this from four words. four IMPROVISED words.