Natura

Once upon a time, there was an everyday college student named Hazel Pomona. The only thing different about Hazel is that she sometimes dabbled in sorcery. Otherwise, she was completely normal: running to class, hanging out with friends-- especially her best friend, Marika.

One day, Hazel noticed that something seemed off about Marika. She was overly tired, and seemed distracted. Hazel figured out that the cause was magical in origin (something she kept secret from Marika, who had no idea Hazel was a part-time sorceress). She went over to Marika's house with her most powerful magical artifact, a ten foot tall staff made of Rosewood, which would filter through the magical energies and allow Hazel to pinpoint what the source of Marika's threat was. It turns out that the cause of the danger was a powerful entity Hazel had never seen before, something made purely out of nature magic. It was so powerful it began eating through the staff. Hazel had no hope of containing it. She wracked her brain for some way to save Marika from this entity, but as time passed, the staff got shorter and shorter, pushing Hazel to the ground. It was going to kill her by smothering her if she didn't do something quick. She realized the only way to save Marika, and herself, was to accept the power into her own body and hopefully contain it.

Hazel was now a shell for Natura-- a millennia old being who had been without a body for centuries-- and Natura was in charge. She quit college, broke off ties with all of Hazel's friends, and set off in search of a way to make her vessel more powerful.

TL;DR version: phenomenal cosmic power, itty bitty living space Powerful nature magic supervillain trapped in a mild-mannered hero's body. In D&D terms, Hazel was a level four wizard trying to contain a level fifteen being. While Natura is indeed powerful, her current situation limits her ability to use that power.
Also, in more D&D terms, Hazel is Neutral Good while Natura is Chaotic Neutral.