Iirc, she is camellia's half sister and lover of the hero in the group that was summoned. The hero was treated much better than the rest of the students, and was the most torn when choosing between the kingdom and his classmates, when it was revealed to him the mistreatment they went through.
Celia is portrayed as a scheming, sly villain by her sister, but from what I remember she is just someone who doesn't like what her family is doing and genuinely loves the "hero" from MC's world.
After the MC and his classmates fled, Celia was imprisoned for helping them, since the class was meant to be used as slave soldiers.