Sunlight and Illumination have nearly identical functionality, with the only differences being who (Leona's allies and Lux herself, respectively) and what (any allied damage source and Lux's basic attacks, respectively) triggers them.
"Brilliance in pieces, shrouded in ethereal flame. Protector, light bringer. Lone voice. Stand and fight. In death-heavy fields, now still she stands, light washes over... will sacrifice anything to protect."
Long before they became Aspects, Leona and Atreus fought each other to a standstill when they were kids.
Leona is searching for Diana, first for zealous revenge against the Aspect of the Moon for murdering the Solari elders but ultimately to uncover the truths the Celestial Aspect of the Sun shared with her on Targon's summit.