As I was working out my role, I realized that in the nominal time period of our roleplay "samurai" was just a rank of civilian public servants - they didn't really appear as a warrior class until the 12th century. So Genji will just have to content himself with being a foreign sword-for-hire and use a chokuto instead of a katana *laughs*.

As to his reason for being in Chang'an -- well, at this period, Japan was courting and absorbing Chinese influences like mad. Many Japanese went to study Chinese science, technology and culture in Chang'an, and the Nara court sent Kentō-shi (imperial embassies) to the Tang court. Genji arrived in an envoy's retinue on one of these diplomatic missions, and got left behind when the rest of the party returned.