In Fallout 2, Charisma also determines the number of base companion slots the player character is given. This number is equal to their Charisma score divided by two, rounded down. As an example, with a Charisma at five, two can be recruited. Please note that one companion can still be recruited (in a few cases) even with the lowest Charisma score.
Predesigned Primary Charisma-based Characters (PPCC) of Fallout are Albert, and in Fallout 2's case, Chitsa.
In Fallout Tactics, Charisma is, at best, a secondary trait. Due to the nature of the game (i.e. almost exclusively combat-based) it is feasible to complete the game easily with 1 or 2 Charisma with no penalty. The only role is that when the Warrior starts with Charisma of 10, their rank is Senior Initiate, rather than Initiate. It is involved with the Barter skill and promotions, and is required for the Leader and Divine Favor perks. A low Charisma, on the other hand, gives access to the Loner perk.
Charisma increases the disposition of all NPCs, which makes Speech checks easier. Disposition is also affected somewhat by Karma (most NPCs like good characters more), quests (doing a quest in a way an NPC likes can dramatically boost disposition), and dialog options (being a jerk in conversations can lower disposition a little). Charisma also affects the Barter and Speech skills. Characters who use Speech will want at least a decent Charisma, but adding points to the skill is almost always better than adding points to the score. Characters who do not use the Speech skill have little use for this SPECIAL score.
There are also general Charisma checks in dialogue with some characters, in addition to Speech checks. Avoid wearing things like Enclave Power Helmets, as they deduct 1 point of Charisma while equipped.
Charisma in New Vegas modifies Barter, Speech and Companion Nerve. Companion Nerve gives each companion in the party +5% to their damage and armor, up to a maximum of +50% at 10 Charisma.
Regardless of Barter or Speech level, some interactions will not be available with a too low Charisma (Ex. Corporal Betsy). These ultimately do not have any significant effect on game play, nor do they provide any special bonuses.
If the Courier's Charisma stat is the most extreme (highest or lowest), Doc Mitchell will either comment "them bullets didn't affect your charm none." (for highest) or about frontal lobe damage. (For lowest)