Soraka is a fallen celestial being, a Support champion on the Summoner's Rift, where she always rushes to the aid of her teammates.  She gave up her immortality to protect the mortal races from their own more violent instincts.
Compassion and mercy are some of her key attributes, but she can also be described as kind, loyal, protective and caring.

Here's her in-game voice for reference: https://www.youtube.com/watch?v=fkhZo3KTeV4
(I chose the audition lines from there as well, but feel free to use your creativity and capture her personality your own way!)