Elementalist Lux has 10 forms: Light, Water, Fire, Air, Nature, Ice, Storm, Magma, Mystic and Dark.
After building up energy via attacking enemies, Lux can change her forms twice throughout the game. You start the game as the Light version of Elementalist Lux, and you then get to pick between Fire, Nature, Water or Air. Later on in the game, you can transform again, combining the elements to get her final form. The elements mix, so if you want Magma, for example, you have to combine Earth and Fire.
What are the combinations?
Magma - Fire + Nature
Storm - Fire + Air
Mystic - Water + Nature
Ice - Water + Air
Dark - Nature + Air OR Water + Fire