Looking to enter ASAP the prestige class kensai (Complete Warrior 49-52). It looks like a fighter could enter the class except for the required skill ranks. Is there a way—maybe by using regional feats—to get Concentration and Diplomacy as a class skills? Or are there other classes that more conveniently enter kensai quickly?
There are numerous ways to get the skills you need without sacrificing base attack bonus.
Hexblade, also from Complete Warrior, has full base attack bonus and Concentration, Diplomacy, and Ride as class skills. It’s fairly mediocre, but you get arcane resistance (like divine grace but only against spells and spell-like abilities) and you can get the dark companion (Player’s Handbook II) instead of a familiar, and those are pretty good. The 5th-level bonus feat is, unfortunately, useless, since it has to do with the hexblade’s (awful) spellcasting.
Paladins in other alignments are possible: Unearthed Arcana has the LE paladin of tyranny. Dragon vol. 310 has the LN enforcer and vol. 312 has the LE despot. All of these have both Concentration and Diplomacy, though the enforcer and despot lose out on Ride (no great loss since you can just multiclass to get that; plenty of full-BAB classes have Ride in class).
Other ranger combat styles are available, for example the Strong Arm style in Dragon vol. 326 grants Power Attack.
Dragon vol. 310 has a number of variant fighters: one of those is actually called kensai, and receives Concentration as a class skill. It doesn’t get Diplomacy or Ride, though. Many of the other variants do get those, but not Concentration.
The crusader and warblade from Tome of Battle both get Concentration and Diplomacy as class skills, and unlike everything else mentioned here, they get 4+Int skill points per level. The crusader also gets Ride. On top of that, they’re excellent classes—by far my preference among those listed here.
The Martial Study feat from Tome of Battle teaches you one of the maneuvers from that book, and also makes the skill associated with that maneuver’s discipline into a class skill for you, always and regardless of class. Concentration is the key skill for Diamond Mind, so anyone can make it a class skill by taking Martial Study and selecting a Diamond Mind maneuver—moment of perfect mind is recommended. And then Diplomacy is a key skill for White Raven, so another Martial Study could get that—white raven tactics is particularly good, though there are a lot of options. And Martial Study is a fighter bonus feat.