I'm pondering this as I'm writing because they have different ambitions, different perspectives yet to me somehow they feel the same. It's not the way they act, but rather when they're there on scene they just feel like a collective rather than individuals to me. Which they shouldn't because: A) Owes life debt to B, averse to killing, has his own ideas of right and wrong which gradually become warped under B. B.) Not so much sociopathic as rather understands what he needs to do to stay alive and feels no remorse about what he does, has a rather negative mindset. Gradually influences A throughout the course of the novel til a certain point. C.) In a way she's very nihilistic, the whole point of the journey to her is to find a sense of purpose or at least some affirmation of her existence. The only possible reason I can think of is that they're all jaded/pessimistic in their own ways, yet this negativity kind of resides in all of them which might be how they feel the same? Because when I'm writing them, I'm writing them from the same mindset perhaps even if their own motives/personalities are different?
To me it sounds like you could be thinking too "directly" about these characters. What I mean is that perhaps when you're writing or outlining your story and get to a point in the story where a character has to make a decision you make their choices for them rather than getting into their respective mindsets and consider what they're personalities and experiences would make them do in that situation. Because you clearly know of some important differences between your characters as shown above it seems to me that you're just not executing that correctly. Your characters are different from eachother and have their separate parts to play, so if you could only internalise that, everything would be perfect. To make sure they are each clear in your mind in their own right I recommend you make sure to memorise their full names, origins, appearance and maybe a couple of central traits to each character. If they're all white, blond, tall, smart or pessimistic, don't focus on those characteristics. If you manage to pin-point the key differences, their remaining traits can simply be kept on a piece of paper or something for consultation when your characters need to make decisions. Also try to remove yourself from your characters. They don't do what you would do because they're not you. If you can find a spark or a kernel of something that made them who they are, you can just run with that initial idea: if they were abused as a child they might not be able to trust people and this might lead to them being cynical, and independent yet shy at the same time whereas if they were spoilt they might be very naïve, and so on. Don't despair, by the way, because as long as your characters appear as interesting characters in their own right in your story when read by your readers, it doesn't matter if they blend together for you, and, besides, by the time you've finished your story, you might not have this problem anymore. My last advice is that you don't change your characters much (particularly their names) after the initial developing stage of a story idea as this might be very confusing. It's best to decide who your characters are as soon as possible. Good luck. NB: These are just things that have worked for me. If my advice don't cut it for you, hopefully someone else will come along with something that's more specifically about your situation.
