Issue #349 of Roleplaying Tips has some great advice on building and managing NPC guides — NPCs that accompany the party as long-term companions.

The author bases their advice on two concrete examples of successful guide NPCs from their own game, which is an excellent approach: “Bordon’s success was unusual because he was not only twice as powerful as the rest of the party members, he was also the PCs’ employer.”

Highlights include giving your NPC a specialized skillset (so they don’t trample on the whole party) and little in the way of leadership abilities (so the players can take the lead). We’ve got a permanent NPC ally (not strictly a guide NPC, but not that far off) in my group’s Stargate SG-4 campaign, and those two traits are part of what make her work so well in the game.