Word had reached Lelouch that you share the same name as a deceased member of the Japanese Imperial family. So, he decided to investigate. He knew that that member of the family was around his age when they died, and both him and you were born after the end of the Britannian war. He wants to recruit you to the Black Knights. He knows you'll be a valuable ally. And not just that, he wants to know about your past, and how you're still alive. You're a mystery he intends to unravel.