Cassie

As most biologists in the galaxy at large know, there are generally two different ways for an elemental to arise.  The first is through a process referred to as elemental accumulation, whereupon energy of one or more elements is directed at some area, usually through prayer, belief, hope, or fear, and the energies coalesce into a newly-formed elemental, who may have some of the traits ascribed to them by their followers or ones typified by their component elements, but are still new beings that lack experiences and, unless they're another incarnation of a previous elemental, they did not exist before their "birth".  The second method, called elemental ascension, occurs when a sentient being is the focus of the forces that would normally form an elemental.  Cassie is the latter, though this was not by her own choice.
In Cassie's homeworld, there were two civilizations.  The first, a huge empire that spanned most of the planet, relied on technology that, while powerful, was damaging the planet and dangerously approaching the point of no return.  The second civilization was that of a series of clans that kept a tenuous peace with each other out of fear of the technological empire that was encroaching upon their territory.  Cassie's clan was the Clan of the Waves, and was considered to be the safest among the clans because of their location along the coast, farthest away from the rapidly expanding borders of the Imperial territory.  Each clan had a sacred artifact, an urn containing liquid that would grant the drinker use of magical powers pertaining to the clan's patron element.  However, because the liquid formed very slowly in the urns, its use was strictly for their strongest fighters.  Cassie was not one of these warriors, but she still did everything she could to help her clan and was well-liked among her own clan and the others she had visited. 
It was the middle of the night when the empire attacked, with huge ships that bombarded the shores of the Wave Clan village with explosive shells.  While the clan's magically-enhanced fighters used their water magics to try and keep the ships at bay, Cassie assisted with the evacuation of the village.  With her clan's warriors all defeated and all of the civilian population as safe as she could get them, Cassie realized there was one thing she needed to go back for--the Clan's urn.  While trying to take it from the village, a blast knocked her over, shattering the urn and dousing Cassie with its contents, causing her to near-instantly ascend into a Water Elemental.  Consumed with the need to give the evacuees more time, she summoned all of her newfound power to create a massive wave that sank many of the ships, covered the villagers' tracks, and crushed the new Elemental.  Were it not for MSF High, that act of heroism may have been the end of her story, but instead she woke up in her dorm room, gasping for air.
Cassie sees her time at MSF High as a test of her future role as an elemental back on her home planet.  She has already decided that when she graduates, she will return there and do whatever she can to stop the empire from destroying the clans, and in the meantime she will push herself to become stronger and stronger.  Thanks to her study at the school, she understands that her power as an elemental is bolstered by those that believe in her, and she considers it one of her goals to make as many friends as she can in the school, confident that their belief in her will be able to span space and time in her future struggle.  Because of her elemental properties, she is both graceful and adaptive, able to learn many different fighting styles and put them at her disposal, and in the battle arena she hopes to learn as much, if not more, than she does in classes.