You know who you are. I have at least one of you in every book. You're the characters who weren't really supposed to be there, the characters who latch onto the main cast and hitch a ride with them. Sometimes you force my other characters to learn and grow. Sometimes you're there to add support. Sometimes you seem to be an annoyance to others until they realize you really do play a significant part.
There is a difference, I think, between you and the characters who show up unexpectedly. The unexpected characters are the ones who jump out of nowhere and say, "Hi, I'm joining your story now!" You, on the other hand, are the ones that say, "Hi, I'm a part of your story and now I am going to do this and you can't stop me, ha!" Or, in the cases of some personalities, you may be the, "Hello, may I please join in on this adventure?" kind of people.
In any case, you always add a little something (or a big something) to my stories and I really enjoy seeing how you affect everything.
*Do you have "tagalong" characters? Do you find them frustrating? Helpful? Annoying? Fun?