Trever Vaenic is an enigma that is mentioned in Act 2 but the culmination doesn't happen until Act 4. However, you will need to be careful at all times not to pick wrong choices if you want to save him.
Trever Vaenic is a pretty hard character to save in Pathfinder: Wrath of the Righteous . The good news is that he can be saved but the bad news is that if you're missing the required dialogue option, you will probably have to wait until the next playthrough in order to avoid him going out in a blaze of glory.
These are the steps you will need to take in order to save and recruit Sosiel's brother.
- Do Sosiel's quest in Act 2 to start the chain. It doesn't matter what you say about the cultist that raises the undead
- When your camp gets attacked by gargoyles and taken to the Lost Chapel, talk with Sosiel about the fallen Hellknight who was carrying his brother's shield. Do not sell or drop that shield at any point. Just let it stay on Sosiel's alternate weapon set.
- When you're back at the camp, talk with Sosiel again and make sure to pick "You don't know what your brother has endured in this war. Perhaps the person you once knew is gone.'" dialogue option
- Ask Sosiel what Irabeth told him and then pick "The battlefield changes people. Your brother isn't the first to be hardened by war." in the dialogue
- When Graham the impostor is presented by Anevia, throw him in jail. Talk to him and he will send you on a wild goose chase where you need to fight a giant worm.
- Kill the worm, inspect the dead Hellknights and return to Drezen.
- Speak to Graham again and force him to tell you where the camp of Hellknight deserters is.
- Go to Lost Chapel and speak to the duo.
- Sosiel will start having doubts and make sure you pick "Your brother hoped to keep his heart pure - but he couldn't. Alas, this happens to many people in war." when it appears in the dialogue.
- In Act 4, you will be able to go to the Fleshmarkets and meet the masked demon that abducted Trever. Ask him about the Hellknight's location. You need to do this before you go through the chain of quests for Battlebliss arena.
- Go to the arena and talk to the manager, ask about Trever.
- Accept to arrange the fight between Trever and Sosiel
- Don't attack Trever in the arena. When he attacks, select the option to raise the shield
- When presented with dialogue options, choose "Trever... What have they done to you? No, that's not what matters. You're safe now, its over. I've come for you. Let's go home."
The dialogue option in step #14 is the only one that lets you save and recruit Trever. If you don't get it, you probably missed out on some of the choices in previous acts and will either have to load a previous save or, more logically, pick him up on a different playthrough.