Jade N'guyen, Cheshire, assassin extraordinaire was always, and always will be, extremely protective over Artemis. She wanted her to keep her childlike innocence for as long as she possible. Artemis needn't know how cruel the world was and could be. She was desperate to make sure her little angel could never be corrupted from the dark and the slime of Gotham city.
Jade was always there for her; scaring away bullies before they could even take their first swing with words sharper than nails; taking blows for her during 'training' with their father (which, more often than not, caused her to receive more), working hard and taking on odd jobs to make sure her little angel never had to need or to grovel, she didn't care as to her own wellbeing not as long as Artemis' eyes still lit up as Jade read her Alice in Wonderland for the thousandth time.
She strived to be everything her father needed, everything he ever wanted, so that he would forget about Artemis. "Don't look at her, don't train her, don't hurt her. Look at me, train me, hurt me, I can take it.". It was her little sister's naive hope and childlike curiosity that fueled her, made her strong and, eventually, cold. She became the perfect weapon-guiltless, manipulative, uncaring-for only one reason, to protect her sister.
And when she couldn't face her little angel anymore, when she realized she had become the very monster she had strived to protect her sister from, that she was a danger to the only person who meant anything to her anymore, she left.