Omi didn't seem to have many memories of his own past, except that while saving a kidnapping victim, he began to remember things that he had repressed as a child. He too had been kidnapped as a child, and he remembered begging his father to come and rescue him, as the kidnappers wanted the ransom money... however, his father or money never came, but fortunately, he was rescued by... Persia? Since his father never reclaimed him, Persia gave him the name Tsukiyono Omi and gave him a semi-normal life, one that eventually included joining Weiss. He did more research on the kidnapping and his past, and somehow discovered that there were three, not two, sons of Takatori Reiji... Upon confronting one of the targets, Takatori Hirofumi, he discovered his real name: Takatori Mamoru, the youngest and missing son of Takatori Reiji. It was not until the end, however, and Omi probably never found out, but Reiji revealed to his brother Shuuichi (Persia) that Omi/Mamoru was actually his son (Persia's), which was why he refused to rescue him so long ago...
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!