im trying a multi player game, and i was wondering, is the reaction adjustment applied to all characters when you encounter an NPC? Or just your main character who usually initiates all dialogue? Why waste points on charisma when they can be going to wisdom and improve my saving throws?

I do believe that reactions are based on the character that is on the top of your list. Not necessarily the protagonist. place your pc/npc with the highest Charisma in the top slot when reaction mods apply. i.e. when shopping.