In addition to my last article on the subject, I just wanted to add that its a good idea to speak to your fellow players if your pet NPC is getting a lot of air time. They might have NPC preferences of their own. They might even dislike the one you've grown attached to. They certainly might take issue with dragging another NPC along to steal some of the limelight (even if only a little).
The Storyteller might also find it difficult to roleplay an ally and your enemies at the same time. I know I can find it difficult to switch mentalities between trying to lay obstacles at your feet and trying to help you past them without giving the game away.
This doesn't mean you can't do it. Just be sure to talk to the other players and check their feelings on the subject. Generally, even if the players aren't keen on that NPC they may be willing to make compromises. A child NPC who stays with the camp rather than entering the dungeon. A girlfriend who handles a lot of the legwork that the team doesn't want to. Or perhaps if its visiting them that's the problem, you just don't go and see them quite as often or at least don't roleplay it every time you do.
Generally, most players are happy to compromise if you will. Also, if you ask they may have an annoyance that disappears by the very act of your asking. This is most likely the case if they felt that your NPC attachment was a sign of your lack of interest in, or care for, the other characters or how the other players felt about the situation. Just by checking with them, you can allay their fears and the issue vanishes.
Just a thought.
Have you ever experienced the irritating or painful side of NPC Adoption?