How does introducing this character into the story actually enhance it? Who controls the character? Is it just an excuse for you to "play" as a PC while being the DM? Do other players have similar levels of attention when it comes to extra story bits?
I'd seriously consider just asking the player what they think. These kinds of ideas sometimes sound great or funny on paper, but then you end up pissing the player off because now there's some external crap going on with how their character is playing out -- possibly in direct contradiction to what they were intending when they made the character.
It almost feels like you're forcing a character story onto the player, though, so don't be surprised to find out they aren't interested. And if you do go through with it, and they "reject" the NPC, you'll possibly make things worse by trying to force her into a more prominent role with the story if it's done in a way that make the player feel like it's the *only* character development he's getting to do.
But if the player is totally cool with it or something, then go for it.
Player is all for the idea, and it would probably function similar to a familiar from the Find Familiar spell, but without the arcane aspects. Few actions of actual use in combat, or maybe the ability to swing a sword, but no proficiency in any weapons. I would probably let the player control it, unless she didn't want to, in which case I would.
The other player has, regrettably, not given me much on a character story yet. I have no idea when we will be playing though, so he has time. As for what she adds, the PC is an official Knight, and I had already planned on letting her acquire a squire at some point, to train on his/her way to becoming a Knight. This gives me a fun way to do that, without it just being "you get a squire". As for the rejection, I am still not sure that is how I would handle it, just one of the ways I could see it going, depending on how badly the player breaks it off. Sitting down for a long talk about the dangers of the wide world, and how the girl isn't ready for them, would probably turn out fine. Sneaking away to abandon her in the middle of the night, not so much, and might lead to a bit of understandable anger, depending on the conclusions drawn by the girl.
I am mostly on here to ask about the mechanics end. I can probably wing it, maybe with Commoner stats and proficiency with one tool set, sewing, chef's tools, that kind of thing, but I wanted to see if anyone else had prior experience on this.