The circumstances in which Zerok joined the Chain of Acheron are left unknown.
“Red Sky at Morning” (Episode 1)
“Sailors on the Sea of Stars” (Episode 3)
The Rosso Cielo was ambushed by the Somnium Tenebris, leading to a boarding party. As the senior officers enter the Somnium Tenebris, Bigcat growls at Zerok, knowing something is off about the "dwarf". Commander Sweet had a suspicion about Zerok but had no time to act.
“The God Factor” (Episode 5)
As the Chain of Acheron takes their supplies to the Somnium Tenebris, Angel finds the dead body of Zerok stuffed into a crate. He takes Judge to see the body and explains that the body is days old but he saw Zerok among the rank-in-file soldiers (the same dwarf Bigcat growled at). The two come to a conclusion that this is the spy Slim was talking about and is capable of changing their appearance.