Runescape invites you to create your own backstory. It was born out of tabletop roleplaying games and the like, where backstories and things are completely up to you.
Diamondgreen was born in Falador to a devoutly Saradominist family. At a young age she struck out on her own, hoping to make a living fishing, cooking and relaxing. A love of learning eventually led her to the Wizard's Tower, where she learned a lot about magic and discovered a passion for runecraft in particular.
Your backstory is your own, and Jagex have already said they plan to keep it that way. An official backstory would make the character theirs, and not yours.
23-Feb-2016 13:05:41