The thing is that the dynamic between John and James does sound genuinely fascinating from the glimpses we get of it.
John says that James was always better at (social) stuff. James did go out and become a police officer, marry Lucy, and just generally have more of a "normal" life, but it's clear that Lucy thinks John and James *aren't* as dissimilar as John believes. James may have been better at masking, but he clearly didn't have an easy ride either. Still, John is envious of his brother and feels like James is what he *could* have been if he'd been able to venture out into the world. The fact that both brothers had crushes on Lucy only serves to hammer that in.
On the other hand, John is an incredibly successful puzzle solver and spending most of his life in his house in no way detracts from that. The murders he solves appear to be beyond the capabilities of James going by the reactions of everyone else. There's an argument to be had about how much is difference in solving vs being tied in by procedure and social niceties, but either way, John gets accurate results much faster than James ever did. Even if John does very much ignore behavioural factors as he attempts to simplify each crime scene to a "puzzle to be solved".
There's also something to be said for how James' coworkers didn't clock he had been replaced. How none of them even knew he *had* a twin brother. Obviously, there's an element of it being down to the writing in that the game couldn't be given away too quickly, but it does speak of a more distant relationship with his coworkers (perhaps aside from his former partner). And there's the voicemail where James says (though given the context, this can't be taken as 100% true) that he was the one most like their father, in his choice to run away. It was a hint to John, but it only worked as such because they had so little that tied them together in the first place. They grew up together, shared their childhood, and yet, they hadn't spoken in years. Makes me wonder how much of it was neither wanting to be the first to reach out. On John's side that makes total sense (James is the "social successful" one, after all) but James' side is much more a mystery (though, "they're more alike than they seem" comes to mind).
Of course, both of them were impacted by their father leaving as he did. It's spelled out several times that John and James reacted differently. John turning inwards (only pushed further by his bullies and discovering his love of puzzles) whilst James focused outwards (to become more socially successful). But we find that John doesn't want to lose the social aspects once he's found a place for himself. That James is able to go almost entirely non-contact with his family despite knowing the impact his own father had on his family. The one stuck in his house ventures out as the one surrounded by family escapes it all.
All in all, John and James are pretty interesting characters even if we only know one of them secondhand.
*However*, when we actually *see* James in person, my main thought is "that's a second David Mitchell"
