Ping-Cho was a sixteen year old Chinese girl whose father was a government official in Samarkand. In 1289, she travelled with Marco Polo to Peking, where she was to marry an elderly member of Kublai Khan's court.
When the First Doctor and his companions joined Polo's group, Ping-Cho and Susan became good friends. One evening in the Gobi Desert, they followed the warlord Tegana and were caught in a sandstorm, from which Tegana rescued them.
When Polo became suspicious of the Doctor's group, he separated Ping-Cho and Susan, fearing Susan's influence on her. Ping-Cho noticed where Polo had hidden the TARDIS key and was instructed not to tell the others. She didn't break her word, but she did retrieve the key herself and give it to Susan.
As the caravan travelled, Ping-Cho became more and more reluctant to marry an elderly man whom she hadn't met. She was interested in Ling-Tau, one of Khan's messengers.
When the group finally arrived at Peking, Ping-Cho learned that her intended husband had died before she could even meet him. She chose to stay in Peking rather than return to Samarkand. (TV: Marco Polo)