Runner 5's double, also known as "Zombie Runner 5" or "Zombie 5", was a Ministry of Recovery soldier sent by Sigrid to impersonate Runner 5 after their helicopter was shot down in S1M1 (Jolly Alpha Five Niner).
The double was with Sigrid and Moonchild outside Abel Township when Runner 5 was sent there in the first mission of the series, intending to replace them and infiltrate the labs underneath Janine's farmhouse. They wore a Mullins Base uniform and latex makeup to match their appearance exactly so no one would be able to tell the difference, as well as carried a Mullins ID with the letters 'Z-92' written on the back. When Runner 5 managed to survive the helicopter crash by jumping out with a parachute, the original plan had to be abandoned.
Sometime between S1M1 and S4M22 (Runaway Baby) , the double died and was turned into a semi-sentient zombie by Sigrid.
The double was first introduced in S4M22 as a semi-sentient zombie who attacks Steve Sissay and steals baby Sara. It runs off, being pursued by Five, Owen , and Sam , to a nearby school where it uses a key card and code to enter the building. Sam manages to follow it inside while Five and Owen hold the security door open.
Sam chases the zombie to a room with a cage in the center and manages to wrestle baby Sara from it. He then gives baby Sara to the real Runner 5 before getting bit by Zombie 5.
In S4M25 Five, Steve, and Dr. Lobatse go back to the building Zombie 5 was last seen in an attempt to track it down and bring a sample back to Abel. The three of them chase the zombie and corner it inside the school. While they're trying to figure out how to bring it back to Abel, Steve kills it and tries to get Dr. Lobatse and Five to leave with him. Dr. Lobatse ignores him, as they need a sample from it for Veronica, and uses a bone saw to cut off its hand. She gives the hand to Five, who bags it, and the two of them head back to Abel.
Veronica injects a blood sample from Zombie 5's hand into Sam at the same bite site, using his natural immunity to create more anti-bodies against the zombie virus. She then takes a blood sample from Sam and injects it into Paula in an attempt to save her life.
It's speculated by Sam and Janine that after Runner 5 survived the helicopter crash, Runner 5 was supposed to die while helping distract zombies while Maxine was in labor. Zombie 5 was then meant to convince everyone else that they had been bitten and stole baby Sara themselves.