I would only allow familiars to progress like characters (gain skills or abilities) if the main character spent a significant amount of their own experience to give their familiar the skill/ability in question. The price would be high, and the character would have to keep track of how much experience was used to improve their familiar (which of course would not count to the main character's level).
Alternatively, if someone wished to play the familiar as if it were bound to the other player then maybe it could be allowed. In such a case it would be treated as a henchman. Usually, gaining only a fraction of experience of the main character. This has benefits and drawbacks. The familiar's player it would never be overlooked, as there would be a live person representing them. It would, as it should, have a mind of its own (both a good and bad thing) allowing it to participate more actively in an adventure. The player of the familiar would be bound to the charcter player, so its autonomy would be limited to being completely honest to the character (no prevarication, omission of facts, misleading, or mis-directing) and would have to obey all direct orders from the character regardless of situation. "I order you," or "I command you," will have to be clearly stated in such situations (this should be a relatively rare occurance subject to DM adjudication).
Alternatively, if someone wished to play the familiar as if it were bound to the other player then maybe it could be allowed. In such a case it would be treated as a henchman. Usually, gaining only a fraction of experience of the main character. This has benefits and drawbacks. The familiar's player it would never be overlooked, as there would be a live person representing them. It would, as it should, have a mind of its own (both a good and bad thing) allowing it to participate more actively in an adventure. The player of the familiar would be bound to the charcter player, so its autonomy would be limited to being completely honest to the character (no prevarication, omission of facts, misleading, or mis-directing) and would have to obey all direct orders from the character regardless of situation. "I order you," or "I command you," will have to be clearly stated in such situations (this should be a relatively rare occurance subject to DM adjudication).
