Friendship affinity

BP is your number of Brave Points. The number of BP you have influences your performance in battle. For me, it maxes out at 3 BP.

Certain actions naturally consume BP, and others are fueled by it explicitly (like Valkyrie Acrobatics, for instance). Some accessories and skills allow you do more damage and take less with a higher BP. Having a negative BP makes you more vulnerable in any situation.

The Brave menu command spends 1 BP to give you an extra move in battle. The Default command has you defend and you get +1 BP at the end of the turn.

If you start a turn with negative BP, you can't act. Negative BP is automatically incremented at the start of each turn.

Reactive abilities like Counter are not inhibited by negative BP.

You can easily check the base stats of each character under the "status" menu, in "tactics."

I'm currently playing through at level 13, so my stats may differ from yours; however, the base values for my current party are as follows:

Tiz: Str 16 Vit 16 Int 15 Mnd 16 Dex 16 Agi 14

Agnes: Str 14 Vit 14 Int 17 Mnd 17 Dex 14 Agi 13

Ringabel: Str 15 vit 15 Int 16 Mnd 14 Dex 17 Agi 17

Edea: Str 17 Vit 17 Int 14 Mnd 15 Dex 15 Agi 14

So as you can see, Edea is suited to be a fighter-type; Ringabel, to be a faster type, a la thief or ninja; Agnes is best as a mage; and Tiz is an all-rounder with slight leanings toward Mind-type or Strength-type jobs.

Edit to add: However, the other poster IS indeed correct that the difference is pretty small, and mostly insignificant. As a minmaxer, though, the difference between 17 base Dex and 14 base Dex is pretty huge early in the game.