One might notice Alex's apparent inclination to prefer conjuring up fire over other spells. From that, one may make the hasty assumption that Alex is a "Fire Mage" or something similar. This is quite untrue, given the DoM universe itself. Unlike other various magic universes, mages and magic are not generally classified by element. Sure, fire is still fire, water is still water, etc, but there are many other fine points one must account for with magic. For example, which "element" would something like a simple movement spell belong to? In theory, a mage could move a boulder via collateral force from wind, flowing water, ice (when propelled as a physical object), or fire, or, as is most commonly the case, direct movement via magic force. As there are many spells which fall out of the traditional definitions of the elements, classifying them as such makes no sense in the DoM.

As for why Alex uses fire a lot, it's not her fault fire is so useful.