@icekatze
Keep in mind that from all we know, she was a manufactured weapon by her sperm donor (the older brother to the Emperor and the ex-emperor) and mummy to drain the powers from the Emperor for his evil(-ish) oppa. So, it isn't unreasonable to see this magical bioweapon as not-quite all-right.
Remember the flashforward sequence of "after-game" when Claude has his powers sapped, the brother's eyes magically are at full power (as Claude's fade and Claude writhes in pain) as he drags his daughter off and she follows obediently like some kind of mindless doll rather than the loving daughter she seemingly was up until then? It plays off like something out of
The Legend of Heroes: Trails in the Sky and related series with
Ouroboros.
Are we sure that she's entirely human or is she messed with? Will she despise herself and break free of "dear old Dad's" control? Will she hurt her cousin? Will she get destroyed by her cousin's boyfriend?
There are so many unanswered questions that could alter matters.
I don't think her "lack" of characterization is without intention.
I suggest rereading and focus on the subtle nuances of Jeannette and her evil sperm donor. Especially, the foreshadowing and hints. Much is given away (if you know what to look for).
I posted on this earlier but it's worth noting again—Jeanette appears to be a bioweapon designed to steal Claude's magic for his brother who was always jealous and angry because he never had it. The Fates seemed to want them to work together, but instead, the brother used the darkest of magics to create Jeannette while also somehow surviving his own impending death by Claude (unlike Claude's ex/Jeannette's mother). As such, Jeannette is an unusual (and largely tragic) character whose purpose is more like an elaborate A.I. doll. She's designed to get close to Claude, unlike Claude's true daughter.
However, Claude & Jeannette's story changed due to the Fates dragging the MC into the story and changing her role. Suddenly the
real/true daughter is in the mix and in a meaningful way, thus making a stylistic contrast to the strangely fake/clingy bioweapon cousin. We don't know yet if her sperm donor Daddy has some kind of "switch" that turns her obedient evil or something else once the yandere gets his brother's magic.
Unfortunately for the brother's machinations, Claude incidentally gave his real daughter (the MC) a great deal of his powers in order to save her (in the process of doing so). Now, she's more powerful than him and thus is either weak to Jeannette's powers or has by definition weakened Claude's powers enough to screw up his brother's plans.
More is here and more I yet explained.
Try rereading it with this in mind and then look at the contrasts between Jeannette and her cousin. How does it play out? Do she lack characterization out of poor writing or on purpose? Is she doll-like precisely because that is what she is—not quite fully human (or, at the very least, being raised as a weapon w/ those impulses). She's dangerous & old Claude (prior to memory loss, feared that she was going to do something like that & kept Jeannette at a distance. Now, he's lost his memories (possibly due intentionally from the brother's actions to attempt to kill Athy resulting in Claude trying to save Athy), yet having Athy reminds him of her mother (and more drama).
Anyhow, I hope this helps clear up some stuff.