Yohji was once a detective, along with his partner Asuka. They were both investigating the deaths of these girls, which lead them to a mysterious corporation called "Riotto" (Riot). While exploring Riotto, however, they were attacked by guards, and Yohji was injured during their escape. The two decided to split up, but the guards caught Asuka and killed her while Yohji looked on. This may have been the reason why Yohji joined Weiss, but that is still somewhat unclear. Later, he truly believed that one of the group known as "Schrient", Neu, was Asuka somehow brought back to life...
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!