Curt was Walker's third officer on Sanctuary Base 6. When he opened his box of mysterious things, he was possessed and markings from a book appeared on his face. (TS: Tardisode 9)
Behind the scenes
According to REF: The Time Traveller's Almanac, a non-narrative source, when Curt looked inside Walker's box, he turned into a dead, possessed being and wiped out Chenna's mind, killing her.