I would say, generally, to keep it small and give the PC a sidekick. Because there is only one player the lone PC can really drive the story and I've had fun playing a sidekick as DM. For whatever reason, it was far easier for the sidekick to not ever get close to being a DM PC.
I've done this successfully playing an NPC familiar to a wizard character. The PC knew she was in charge, so even though she asked the familiar its opinions, she made all the decisions.I would say, generally, to keep it small and give the PC a sidekick. Because there is only one player the lone PC can really drive the story and I've had fun playing a sidekick as DM. For whatever reason, it was far easier for the sidekick to not ever get close to being a DM PC.