Doctor Moon

The Doctor Moon was an artificial moon which was built by Strackman Lux's grandfather to protect CAL, his youngest daughter who became the Library's computer and was given an imaginary world to play in and control of the Library.

In Charlotte's imaginary world Doctor Moon was in human form and was treating her. He protected her and gave her advice to save the others in the Library. That was until she was in a overload and temporary switched him off. When the Doctor saved River Song into CAL's imaginary world the Doctor Moon was back online once more protecting CAL, River and the rest of the team minus Strackman Lux who had not died in the Library.

(DW: Silence in the Library/ Forest of the Dead)