Brother Six was one of several Cwejen who acted as Straxus' assistants on a mission to 19th century Earth. He adopted the name Septimus after a customer at Straxus Imperial said he reminded her of her son.
On Earth, Septimus and the other Cwejen went mad, warping trace memories from the original Cwej into myths of a "pale god" called "Mister Seven". In the hopes that they could summon the Doctor, Septimus and the other Cwejen conducted acts of murder involving the number seven, which were investigated by Bernice Summerfield and Mycroft Holmes. Septimus eventually turned on Straxus, forcing his regeneration. Benny and an earlier incarnation of Straxus defeated the mad Cwejen, which were taken back to Gallifrey for dissection. (AUDIO: The Adventure of the Diogenes Damsel)