Cellian

 and his sister Ciara were known as the Irish Twins.

Biography
Cellian rarely spoke. While in nurse training, he and Ciara stole pharmaceuticals, and were arrested. Dr. Ingrid Krafchin arranged for their release, but the price for this was their being used as test subjects. Krafchin replaced their blood with Nestene Compound and incorporated Auton weaponry into their bodies, making them living weapons. Part of the process also gave them nanites which repaired any damage to them. (PROSE: Business Unusual)

They were the Pale Man's most trusted employees, and infiltrated the Glasshouse for him. UNIT troops stopped them and the Pale Man from using the technology in the Vault to take over the world. However, they all escaped being caught by UNIT. (PROSE: The Scales of Injustice)

By 1989, Ciara and Cellian were having doubts about the work they were doing for the Pale Man. Melanie Bush was captured and held by SenéNet, but escaped and was chased by the Stalker. The twins followed, but Cellian was attacked by the Stalker and lost his arm before Ciara could kill it. Her unemotional reaction to her brother's severe injury made Ciara realise how much of her humanity she had lost. After warning Mel about Krafchin, she took Cellian away. She hoped they could create a new life somewhere else, and make amends for all the wrongs they had committed. (PROSE: Business Unusual)

Cellian's arm grew back thanks to the nanites in him. By 1993, he and Ciara had settled in the village of Halcham and helped out wherever they could. When the Cylox conflict reached the village, Ciara was fatally injured during the fighting. Due to their psychic connection, Cellian also died shortly thereafter. (PROSE: Instruments of Darkness)