There are several potential solutions...
1) talk with him about that behavior.
1a) bounce him out of the group if he won't stop.
2) embrace the in-character play, and quit trying to impose a story.
3) place a few stories that highlight his fixations, and encourage others to "return the favor" ... in hopes he sees how distorted it might be.
I'd honestly suggest trying #2. It may seem counter-intuitive, but the player is in fact not far out of line for a FATE system game. FATE supports strong player participation in plot creation, both by letting the GM know what buttons they want pushed (via aspects), and by truth creation and declarations.
I do several things to keep the player characters interested and invested in each other.
- At the start of the game, I insist that players coordinate backgrounds (subject to my approval) such that each character know at least one, and preferably two or more, of the other characters. In general, I prefer these connections be positive; the most negative I will usually tolerate is on the order of a friendly rivalry. In general, I also prefer that if you start from any one character, you can get to any other character by following these pre-arranged links. (In graph theory language, the players are all connected, although indirect connections are fine; the alternative would be two more more sub-groups connected internally but not to each other.) And finally, I try to ensure that each connection is more than trivial, but not necessarily life-binding.
So for instance, "We met in a bar twenty years ago and never saw each other again," is trivial. "We are cousins who are best friends and we are rarely separated," is life-binding and more than I look for (although it's fine if that's what they want.) Things like the following are what I look for, and/or what I've seen in the past:
- Our characters served in the same unit years ago, and knew each other, but haven't kept in contact...
- I served as a mercenary escort once, while he was travelling with his master from here to there; along the way, this happened....
- We weathered the siege/plague/earthquake of wherever together some time back....
Now, some players are genuinely not wired that way-- if you ask them for backstory, they freeze; if you given the one, they can't connect to it. When I run into a player like that, I have to respect that, but I try very hard to get everyone to adhere to the guidelines.
That does not directly solve the problem. (It actually solves the problem of getting the characters all on the same page at the start of the game.) But it does often give me enough to work with to do the following:
- With enough insight into character backgrounds, and with overlapping backgrounds, I try to give every character an mid-term to long-term goal or plot arc, and then I try to modulate that by giving at least one other character a minor to moderate interest in how the first character's arc plays out.
It's important (to me, for the games I want to run) that these arcs not be strictly opposing: If one character has sworn blood-vengeance on an NPC, I won't give another character the goal of keeping that NPC alive. But I might give another character the goal of getting something from that NPC before his death, or getting the NPC to do something, etc.
And I also try to modulate this in another direction by giving other characters-- ideally, not the same one-- influence over the plot lines. So continuing that thought:
- Player A has sworn to kill Sir Odious, his parents' killer
- Sir Odious has information that will help Player B in her quest to do something else
- Player C knows someone who can be bribed into giving up information about where Sir Odious will be
In that way, for each of the various player sub-quests going on, at least one or two others will be involved somehow, even if only at the periphery. Ideally, Player B has some motivation for something to happen, and Player C has something he needs-- something at least moderately costly or risky. They are invested.
One thing I would not do-- at least not again-- is what you tried:
I've had players create characters (with backgrounds) completely secretly from each other with the hopes of allowing the character interaction to be heavily role-played at the table. Didn't work because of very incompatible characters.
I've never done that, specifically, but I've inadvertently done similar things and it never worked well. It seems like it should work, especially if you pattern it similar to what I've outlined above, but there's a structural weakness to it: If the players, starting out with the relative blindness of only knowing their little part of the background, they just might not see those connections you built in for them, and won't give themselves the incentive to start sharing information. And if your players were the sort that would do that naturally, you wouldn't have to go through these acrobatics in the first place.
I call this "Open World" versus "Story Arc." Obviously both styles are fine but I've seen this problem as well. The sides get a little tired of each other.
If you want to bring your wanderers into the story I suggest you tie some plot points to their characters' history. Or rather, add their history to the plot points.
Examples are to add in mid-crawl bosses that insulted/betrayed them earlier. Or scour their backstories for ideas. Did they know their uncle was a necromancer? That he stole your grandfather's shield from your father? That sort of thing.
The other way I handle this is by using the open-world players to advance the story. I've always felt these folks need to know the whole world is lush and interactive.
So, if they drop the quest to find the dragon's lair to get into the slave trade they'll naturally meet someone that begs for freedom in exchange for showing them a vast treasure. Guess where that treasure is?
Look, people are of course going to get wise to this sort of thing but we all have a tendency to like stories that tie together. Some folks just don't want to follow what feels like a paved road.