Yeah so build the NPC as a hero and just add it as per normal
If you don't want then controlled by the player in battle find and tick the computer control option, oh and lock equipment
Or if you want them as a party member but not on the "party" have a message displayed saying "NPC joined the party" or something and turn on a switch to symbolise this. so any events that react to the NPC on the party only have to look for to switch being on, rather the NPC on party, as the previous method