Permanent companions usually have prerequisites and may not join your party unless certain conditions are met; these vary between potential companion NPCs and often there are multiple possible ways to obtain them as a party member. The game will not allow having more than a certain number of active party members; this limit is half the player character's Charisma, rounded down. For example, a Charisma level of 4 allows taking 2 companions. With the perk Magnetic Personality the Chosen One is allowed one additional companion over the current limit.