takayuki is lucky he’s in prison and under watch 24/7, basically. in my canon arle definitely knew factually that takayuki was abusive since the fatui likely have a dossier compiled on sara, and it wasn’t like he kept his abuse secret. she feels a twinge of sympathy for sara on their first meeting, but ofc she doesn’t show it since she has an image to maintain. however, once their relationship progresses and arle sees the effects of takayuki’s abuse with her own eyes she feels absolute pure rage and disgust towards him. she’d tremble with it as her hand traces the thin, straight scars criss-crossing sara’s back and her eyes catch on the way sara’s wings rest at odd angles from being bound for so long. (the only other tengu we’ve seen visually is sasayuri, and all of their artwork has had them with their wings out as if by default. sara, on the other hand, only unfurls her wings occasionally. the implications are hence… unpleasant.) also to some extent i think arle sees at least just a flash of clervie in sara… both were isolated and tormented by their “parent” figure. crucabena singled clervie out by punishing her for speaking up, while takayuki isolated sara by punishing anyone who spoke up for her. it’s unbearably cruel and arle loathes knowing that sara had to suffer through it alone—because though the kingmaking may have ended the way it did, arle at least knew what it meant to be loved before the end. sara had no one and nothing, a lonely bird in a cage. it pains and sickens her, frankly. she considers breaking into the inazuman prisons and killing him herself—it wouldn’t be hard, she’s an assassin after all—but it’s only by sara’s own wish that she holds back. sara wants to bring him to justice, as is her right, and arle respects that decision. nonetheless, she treats sara with a lot more gentleness than one would expect from the fourth harbinger, especially when it comes to touch. arle is observant, always has been since she was a child, and she watches for all of sara’s tiniest expressions whenever they touch. she asks things like, “do you like this, little bird?” or just generally checking in a little more often to make sure sara’s alright with where she’s being touched. arle knows firsthand how this healing comes slow, so she takes it slow with sara as well. at least in this, her little bird will no longer be alone.
