Thermidor was born about 9 months after Giovanni and Sunday's mom had split up, already rubbing salt in in the wound in Sunday's point of view. Giovanni, still hoping for a replacement son, was disappointed that Thermador was another girl. Nonetheless, Giovanni and Amaranthe still were decent parents to her, and Giovanni ended up neglecting Sunday, partially because she reminded him of his ex-wife, and partially because he was so overwhelmed with the new baby causing the girl to start resenting her younger half-sister for getting all the attention. He would treat her like a son regardless, indulging in more "manly" stuff like playing sports with her, hoping that he can have a little tomboy to get along with if he can't have a son.
Finally, a few years pass and Giovanni ended up getting the son he wanted- Ides - while subsequently losing Amaranthe in childbirth and began to neglect Thermidor in the same way he had already been doing with Sunday, giving the elder girl great schadenfreude. Sunday still refused to sympathize with her sister, and only increased her meanspiritedness/bullying mannerisms towards her, leading up to become more silent and withdrawn. Giovanni's neglect in this case wasn't as intentional as much as it was grief-driven, but nonetheless it still occurred and was still affecting Thermidor's mood. Despite this, Thermidor promised to herself that she wouldn't resent Ides in the same way Sunday had resented her to keep a somewhat healthy relationship between her and her younger brother.
Thermidor began to get an interest in exploring with the new feelings she was dealing with of wanting to leave her dreadfully sad household where nobody paid no mind to her. While she attended tradition school, she often skipped classes just to explore weird caves instead because, who really cares about Algebra? On the occasions she is in class, she's trying to draw up maps to kill her boredom. At some point she was nearly attacked by a dog while exploring, causing her to go from being ambivalent about them to disliking the creatures.
(Hope this is okay! I tried to take some inspiration from your original backstory because I did like it but added some sadder twists and more expansion into it but feel free to change it however you like obviously or discard it!)
(Just choose whoever you want of my human/oid OCs that doesn't have a backstory.)