The Price of Miracles
She was once born a child with an incredible gift: not only a natural affinity and early mastery of healing magic, but also the capability to do so with such potenty and unbelievable results, that those who knew her came to dub such feats as "miracles".
However, during the build up of the Seventh Umbral Calamity, she was kidnapped by cultists of Dalamud, and she would be experimented on. Only after escaping and the events of the Calamity would she finally learn, thanks to the Seedseer in Gridania, of the consequences that came with the experiments: From that very moment, when they did those unspeakable things, every time she would perform one of her "Miracles", she would do so at the cost of her lifespan, although nobody know how much.
The Martyr
There was a time after learning of the revelation that she became quite self destructive. She would come to the aid of anyone that would be in need of her "Miracles", and she would perform them with no hesitation, for she believed that if she was fated to die by this curse, then it would be worth throwing her life away to same many others. It is estimated that she might have thrown away a grand total of 10 years of her lifespan doing so.
It was only after meeting H'elio, Harpocrates secondly, and later on everyone else, that she began to value her own life more, and as such she has made incredible strides towards pulling away from her dangerous mindset, although those those thoughts still come up sometimes.
Blinding Punishment
Celica herself is quite the patient and levelheaded woman. However... she does have a tendency to flashbang people with low intensity light spell burts whenever she boils over.