So, here's an idea. I was building one of these last year and I did what Kurald Galain said above, which was to make a Lazylord. It was also a Pixie, as per your idea. So I thought that to avoid hogging the limelight, he would just sit on someone's shoulder and give out free attacks, never moving into a separate square.

Then I realised I could take it further and refluff the Pixie as a possessing spirit, which can't leave its ally's square except to go and possess another ally. It wouldn't take over the ally completely, just kind of hitch a ride in its body and be a voice in its head giving it hints on how to attack.

Sadly I never got to play this. But it could be fun. Especially if a helpful NPC has recently died and you want a way for them to continue to be helpful.