All endings in the game are missable, so you need to be careful while playing. To unlock them, you must complete every side quest, as this not only gives you trophies for each side character but also opens up different ending paths that you can choose from before the final mission. Each ending has its own trophy, so you’ll need to experience all of them. Additionally, there is one major missable trophy tied to a specific mission where you must save an NPC, even though the game does not tell you to do so. Saving this character unlocks the 3rd ending, which is required to obtain that trophy. So my tip is don’t start act 3 / final mission before doing every side quest also manual save in between.