Li was a Chinese boy who helped the Tenth Doctor defeat a stone monkey.
When he was a little boy in 1829, Li witnessed the arrival of the TARDIS and guided the Doctor to his grandfather, Chan Chiu. After Chan Chiu revealed he wasn't fit enough to go after the stone monkey, Li decided to help the Doctor. They found themselves battling Chinese guards, then battling the legendary stone monkey.
After it was defeated, the Doctor and Li went their separate ways. Li longed for another adventure. This encounter with the Doctor stayed with him all his life, and he told his grandchildren stories about the stone monkey and the Doctor. In 1886, the Doctor and Li were reunited for the final time. (COMIC: Reign of the Stone Monkey)