Henry said:My personal recommendation? Autolycus from Hercules the Legendary Journeys.
He's a character that occasionally teams up with Hercules or Xena, but who has his own agenda, who shakes things up, but is always on the side of the heroes. He has his own adventures off-camera, and shows up once in a while in the season as a guest-star.
The once-a-month guy can do the same. He shows up to help the party, you can give him his own weird agenda to take care of on a hidden note, and he can have fun showing up, doing something fun, and leaving at the end, riding into the sunset or just disappearing to do his own thing.
Make your once-a-month guy the "Guest-starring NPC."
That sounds an excellent idea if you're running 1-session "this week's episode" adventures, where things can be wrapped up at the end of the night. If he tells you in advance the nights he'll be there, and you can work him into the campaign (much easier in a city campaign than a multi-month journey through howling wilderness), it should be good.