pawsplay
Hero
So there is an eight point spread between the fighter and the scholar/martial artist/fighter. You can cut that in half just by getting the fighter a magical shield and a ring to match. I am kind of surprised to see Dex 20 and Int 20. I wonder what kind of ability scores the fighter has by comparison, and why he seems to have thousands less in defensive gear.
Looking at some CR 9 monsters, I see that most will require about an 8 or so to hit the fighter, but around a 16 to hit the other character. That seems pretty normal. With buffs, those ACs will go through the roof, but that is just as true of the fighter as the other character. With an AC of 38, a greater air elemental will need a 17 or higher to hit the fighter and a a young blue dragon will need a natural 20. That reflect less on the AC disparity you have described than with the relative ability of equal CR monsters attacking PCs in melee combat and having much chance of hitting when the PCs are fully buffed. Normally, I would expect the PCs to be fully buffed only for more difficult encounters. In a more challenging situation, say against a CR 12 adult green dragon, the dragon needs a 17 just to hit the buffed fighter. That's Pathfinder for you.
So from what I can see, and I hope I am not trivializing your concerns, it would appear to be that the character in question has unusually high ability scores, while the fighter has a fairly normal Dex and much less in the way of magical defensive gear. Without knowing more, I can guess that the character has basically sunk a lot of points or a good roll into Int for a permanent +5 to AC, in exchange for losing a point of BAB, a hit point or two, and ending up two levels further behind in getting fighter or martial artist abilities.
Looking at some CR 9 monsters, I see that most will require about an 8 or so to hit the fighter, but around a 16 to hit the other character. That seems pretty normal. With buffs, those ACs will go through the roof, but that is just as true of the fighter as the other character. With an AC of 38, a greater air elemental will need a 17 or higher to hit the fighter and a a young blue dragon will need a natural 20. That reflect less on the AC disparity you have described than with the relative ability of equal CR monsters attacking PCs in melee combat and having much chance of hitting when the PCs are fully buffed. Normally, I would expect the PCs to be fully buffed only for more difficult encounters. In a more challenging situation, say against a CR 12 adult green dragon, the dragon needs a 17 just to hit the buffed fighter. That's Pathfinder for you.
So from what I can see, and I hope I am not trivializing your concerns, it would appear to be that the character in question has unusually high ability scores, while the fighter has a fairly normal Dex and much less in the way of magical defensive gear. Without knowing more, I can guess that the character has basically sunk a lot of points or a good roll into Int for a permanent +5 to AC, in exchange for losing a point of BAB, a hit point or two, and ending up two levels further behind in getting fighter or martial artist abilities.