Fallout Wiki said:Charisma is one of the Primary Statistics in the SPECIAL character system. It represents a combination of appearance and charm. A high Charisma is important for characters that want to influence people with words. Modifies NPC reactions, and Barter prices.
Modifies: Speech and Barter skills, NPC disposition, party limit.
In Fallout 2, Charisma also determines the number of base companion slots your character is given. This number is equal to your charisma score divided by two, rounded down. For example, if you have 5 Charisma, you can recruit 2 followers.
For most characters the reaction system is entirely forgettable. It is in fact impossible to get a bad reaction even with CH 1 unless you have Childkiller or Berserker, have negative karma, or go out of your way to insult people. The factors involved can be summed up like this:
* Each point of CH: +5
* Each level of the Presence perk: +10
* Karma: +amount
* Childkiller: -10
* Champion: +20
* Berserker: -20
If the sum is 0 or below you get a bad reaction. If it's 51 or above you get a favourable reaction. Everything in between is a neutral reaction. This calculation is made only when you first talk to a critter, although things you say to them may nudge the result in either direction. In the vast majority of cases neither approval nor disapproval has any tangible consequences.