lol well leander was always implied in-game, to have slept around. When he gets you the room key, the bartender says something about it being the usual room, and leander blushes hahah and says it's just for a friend this time. He's a lil hoe but he is single and very charming so! (plus Vere's insane line about him going out back to get a bj). It was implied in one of the information posts that they had done something. Plus, I think one of the lines in-game implies it, but I can't remember. You can find all this bonus information, plus the charts, on their social media. They post on insta, bluesky, and tumblr. I look at their tumblr.
And yes, I believe Kuras has not done something like that with any of the other characters. It's only implied that he's known Vere the longest and that is related to why he hates him lmao. (bc they're both very old beings). But nothing abt them doing it. Kuras has loved other humans, but bro is hella old, so that's valid.