Speaking of personality and motivation and characters named Sylvia, I enjoyed Academy Heroine's Right Diagonal Back Seat recently but one gripe I did have was the abrupt switch in Scarlet's motivation pretty early on.
Her motivation for wanting to be Sylvia's friend at the start was that she was absolutely terrified of death and thought that sticking close to the main heroine would be the safest place to be. The reveal of the unreliable narration and Scarlet's root trauma and PTSD was great and that conflict between her selfish motivations and how she presented herself as the genuine friend with no ulterior motive that Sylvia was longing for was great set up for drama later on that eventually paid off.

But very shortly after that and for the entire rest of the story Scarlet becomes a self-sacrificing martyr who feels that she doesn't belong in this world, burns her own arm to ashes to protect others, barely cares that she only has 3 years left to live because she is more concerned that the people of the story she loves so much need to be saved, is terrified of losing control and hurting the people she loves, right up until the end she is all too willing to burn her own life out to save the others. That's fine too and also makes for enjoyable drama, but it's completely at odds with her initial characterization. If anything it's characterization that makes sense for the original Subject 10 (who is still buried in her subconscious), but not for isekaied Scarlet.