For what it's worth, I've listened to that banter, and while Eder is definitely crushing hard on Iselmyr, it doesn't seem like to her it's anything more than one of her stupid pranks, this time both on Aloth AND Eder. Because when Eder asks what does Aloth think about them together, and she replies that Aloth wants only happiness for her, I honestly lol'd. Heck, he's getting annoyed even when she comes out just to talk to Eder and repeatedly asks him not to encourage her. And after all that she says: "Ooh, yer face, Edér! Only thing finer'n seeing it is feeling this lad's ire!". It's only my interpretation, of course, so take it for what you will.
That being said, I find myself liking Eder a lot less. Not for being fond of Iselmyr, just for being this dense and obviously lacking some basic empathy.
Welp, all more reason for me to suppress her in my playthroughs, I guess (like I imagine her constant yapping in Aloth's head and trying to assume direct control of his body wasn't enough already :D ).
Honestly, I'm not really a fan of Eder in this game, either. He wasn't my favorite in the last game (I found him bland but inoffensive, so I took him anyway), but he's more self-aware, which doesn't work for me. His jokes are trying too hard, and when he brought up finding the...er..."loud woman," I benched him permanently. The timing was pretty awful; I have way bigger issues to worry about. Like, you know, getting my soul back.
On a side note, I prefer "romancing" adventurers that I create, since I come up with backstories for all of them. It's actually kind of sad fun, from a role-playing perspective.