Maybe the rift itself could be seen as an antagonist in a way. It's a force that has taken Owen away from his friends and his normal life. It could be constantly changing and evolving, making it difficult for the Torchwood team to predict its behavior and rescue Owen.
There could also be a rogue element within Torchwood itself that is somehow involved in Owen's being taken by the rift. This person or group might have their own motives, like using Owen's situation to gain power or access to certain Torchwood secrets.
Well, it could be an alien race that has control over the rift and doesn't want Owen to be rescued. They might see him as a threat or an interesting specimen to study.