"Double rainbow? What does it mean...?" and "Well, a 'double rainbow' is a phenomenon of optics that displays a spectrum of light due to the sun shining on droplets of moisture in the atmosphere. Does that explain it?" reference Yosemitebear62's video.
She features ten forms (one per element, each with their own voice-over, particle effects, and Recall animation) but can only use up to three per game (Light, one basic, one final)
Form swapping is irreversible and affects her Elemental Ward skin as well.
Lux is the youngest in noble family Crownguard, born with magical abilities she keeps hidden for fear of being executed or exiled from Demacia.
Lux fears of what may happen if her older brother Garen finds out about her powers (their uncle was killed by a mage, an event that made Garen show magic users both zero tolerance and no mercy)
Galio deems Lux 'the only mage he likes' ("You, young girl person: can you stay and accompany me wherever I go?") and the one that might help him attain prolonged animation periods longer than a battle.