So, me and Esper RP a lot outside of Iwaku, you might of noticed that we intermeshed our character's backstories. We typically do that because well, we are probably going to end up intermeshed so might as well do so at the outset so everyone knows.
For Bronya's magic I was also going to go somewhat symbolic, the two examples I used were blood (she is short tempered, so 'hot-blooded') and things burned by wildfire, bringing the flame that burned them back out, on top of a general mental/physical fatigue for maintaining a flame beyond igniting it.