No secondary statistics are based on Charisma. It does heavily influence the Barter and Speech skills. Charisma is used to determine the maximum number of NPCs that will join your party. You can only have Charisma divided by two, round down, number of party members. It is also important in determining the initial reaction of NPCs to your character. Characters with high Charisma scores will be better liked. The game will compare your character's Charisma to the NPC's Charisma. NPCs with high Charisma scores will be more resistant to the charms of your character.