Aya's history was a mystery for the first half of the series, for his past and connection to Takatori Reiji was revealed very slowly throughout that time. He joined Weiss for revenge- revenge for his murdered family, and his sister Aya, who remained in a coma after a car accident. His parents were bankers that may have known too much about Takatori Reiji's crimes, so he used them as a scapegoat and had them killed in a seeming suicide while Ran and Aya were away. The kids returned just in time to see their house explode, and the man responsible for it trying to escape... However, Ran and Aya were witnesses, so Takatori ordered his driver to run them over. Aya was badly injured, and fell into a coma, but Ran was saved by a mysterious man, who turned out to be Takatori Reiji's brother, Takatori Shuuichi, that convinced Takatori to leave. Aya was in a deep coma, and it seemed that she would never stir... so Ran decided to get revenge upon those who committed these crimes, and took his sister's name as a reminder during his quest for vengeance.
Disclaimer: Weiß Kreuz is owned by... uh... I believe it's Takehito Koyasu, and Project Weiß, and some other companies as well. It's also licensed by Media Blasters in the US. The pictures on this page are the ones I scanned myself from artbooks and other things- I'd appreciate it if they weren't stolen/used w/out permission! These pages are owned by Chereth but the series is owned by its respective companies. Please don't sue me, link to me instead!