I share that frustration. I hate the miscommunication/ withholding information trope in stories so much! I know that technically Ress doesn't know he has the Demon inside that was made clear at the beginning but he should at the very least share the info he does know for sure. Spoilers ahead!
Like he should have shared that he himself was targeted by the Mercenaries directly. And that somebody wanted to abduct him. He can still say he doesn't know how he escaped. But him trying to investigate on his own is not his job. They have resources and man power to investigate further and more in depth.
Then he should have shared that he saw Red Eyes inside the city and right by the Guardhouse! It is completely reckless not sharing that information. It puts not only them but everyone in the city in danger.
Now comes the more selective truth by the time he figured something really wrong is going on with himself, something demonic. He shouldn't share with everyone but he can and should trust Ludwig with all his info on that. Because first Ludwig already knows the impossible things his body does. And unlike everyone else Ludwig is also affected. If Ress shares with Ludwig that Ress might be possessed he will def want to share with the others. But then he also shares that Ludwig himself might be possessed and then the story might be different. If both are possessed then they both are forced into secrecy because both might be dead if the other Dawn Break soldiers find out. Forcing Ludwig into cooperation and secrecy if he agrees or not.