How Powerful should the NPC be? The NPC should probably be able to take care of itself, but should not be more powerful than the party's PCs. If your NPC is actively hurting the party's enjoyment, you have failed. As for what your NPC Companions personalty is, just be sure that they are not annoying. Every second wasted focusing on them is a second your players would likely rather have focused on their PCs. Be efficient with amount of game time you spend setting up/characterizing your NPC. That being said, giving them too much character can be harmful, too. Obviously, the party is not going to want to spend time with an NPC (let alone allow it to join them) if the NPC is a block of wood. How should I Develop the NPC's Personality? This is probably one of the hardest parts of making a good NPC Companion: giving them a personality without overstepping. An example could be a sailor that the party hires to ferry them across the sea, or a knight that helps the party fight a powerful common enemy. This could be for story reasons or because the NPC brings something to the table that the party needs (preferably both). There has to be an organic reason for the party to accept the help of an NPC Companion. What does an NPC Companion Need? Above all else, an NPC Companion needs to serve a purpose.

That being said, I thought it would be nice to give newer DMs a general set of guidelines if they want to include one in their campaign.

NPC Companions and DMPCs are a hotly debated subject in the D&D community and are really hard to pull off well.

One off npcs dnd me