(I'm no good at creative-writing, so this is just a draft.)
Jade grew up in wealthy Ionian family. A gang of thieves assassinate her parents, but a mysterious ninja saves Jade from them. Jade gets taken under the wing of this ninja (plot twist: it's Zed) who introduces her to the Order of the Shadow. She spends years as one of Zed's students, learning the way of the shadows.
As a final task, Zed assigns Jade to assassinate someone. She completes her mission, but discovers that her victim was an important Ionian figure: a high-ranked Shojin monk. Realising that she had been manipulated, Jade attempts to flee but gets captured by Lee Sin.
Lee Sin takes her to the High Council to be trialled - she was given the option to either a life of imprisonment, or to forever serve the summoners in the League of Legends. And so, to avoid a horrible fate, she joins the league.
Now forced to live under the Institute of War, she meets Janna. They become friends quickly as they share common backgrounds (both are orphans, have similar appearances, etc). Jade never tells Janna about her crime. Janna offers to teach Jade with her knowledge in the elemental magic of air - Jade gladly accepts. With both the arts of Shadow and Wind, Jade aims to fight Zed on the Fields of Justice and prove that she is a more worthy, skilled ninja than him.
"Something, something" - Jade
Young, teenaged girl (older than Annie, but younger than Lux). Has pale skin with light-coloured hair and bright-green eyes. Small, thin, delicate build. Has a 'girly' voice like Lulu but speaks in a quiet, humble tone like Karma.
Wears a dark-robed outfit (similar to Talon's but with a green-black colour scheme). Blades and shurikens strapped to her legs (like Katarina). Wears a traditional Ionian jade-pendant around her neck.
Basic-attack Weapon: Throwing knives
Spell/Secondary Source: Energy
Base movement Speed: 340