Kerko was originally from Rigel 77, from which he escaped with his little brother. He arrived in Splott, where he obtained two pitbullfrogs and started causing trouble. Jack Harkness tried to capture them, but Kerko's brother was accidentally killed. This left Kerko with a grudge against Jack. Jack tried to interrogate him about the recent surge of Rift activity, but Kerko was unwilling to cooperate and tried to kill Jack.
When the Undertaker's Gift was activated, the Torchwood Hub started to break down, allowing Kerko to escape. When Jack returned to the Hub, Kerko tried to kill him in revenge. With the help of a young Vortex Dweller, Jack killed Kerko by electrocuting him. After the Vortex Dweller repaired the damage to time, Kerko was brought back to life and returned to his proper place. (PROSE: The Undertaker's Gift)