i think upon first read (and this applies to all of tlt in general) the arm thing is definitely hard to parse, especially bc ianthe is super closed off and casual, but i really think if you think about it with more depth and the idea that ianthe does actually have access to the full breadth of human emotion, it becomes very clear that it is a metaphor for her grief over naberius. she WILL NOT acknowledge him or his death, and also CANNOT harness his abilities and it is wildly detrimental to her. she has the ability to use the arm in every context other than sword fighting, and that is when it fails her; she chooses not to use the arm in other contexts bc she can’t face her grief! and then she severs it completely, never fully giving into her grief, and then continues to use his body like it’s nothing. we’re told straight up that her persona is calculated so why would this aspect be any different? again. person she has known and served her since birth. they definitely have a deep and complex relationship, it just happens outside of the pov character’s viewpoints.