First thing that came to mind: the dog in Fable III.
Sure, he didn't do much other than help you find buried items that were (for the most part) not worth anything (although on occasion it did pounce on enemies that had lost their footing and finished them for me), but it was still an interesting part of the game.
As an additional note, the caption under the RE5 spot on the list made me laugh.
I can think of several moments when I just let Sheva die (all the other times she did just fine getting herself killed), she's gotta be among the worst AI I've ever had the displeasure of playing cooperatively with.