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Cheen was a female from New Earth, and the partner of Milo.
In the year 5, 000, 000, 053, she and Milo kidnapped Martha Jones, who had arrived with the Tenth Doctor in the Undercity of New New York. They did this to gain access to the Fast Lane of the Motorway, for which cars needed to have three adult passengers. During the trip in the fast lane, their car was attacked by feral Macra. While several Macra tried to grab the car, she and Milo escaped.
At this time Cheen was carrying Milo’s baby boy and was in the early stages of pregnancy.
After The Doctor opened up the Motorway, they dropped Martha off at the New Earth Senate building. (Gridlock)