At the end of Ratatouille, when Remy is finishing his story and says concerning Ego that “he seems very happy”, there’s a rat sitting next to him who says “how do you know?”. Does anyone know who he/she is? Could it possibly be Remy’s mom? I’ve always wondered about this.
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I don’t think it’s anyone important, or we would have been introduced to her.
The screenplay says “teen rat”. So, nobody relevant.
I always wondered who that rat was too, it sounds like a girl… maybe one of Remy’s relatives?
I’ve always thought the rat was a girl, and a young one (“teen rat” sounds appropriate). She’s obviously come into the clan after the story, since she’s listening to Remy tell it as if she didn’t know about it before. (And it makes sense that some time would have passed between the closing of Gusteau’s and the opening of La Ratatouille.)
She’s not Remy’s mother, because there are sketches of Remy’s mother in the “Art of Ratatouille” book. (Her name was Desiree, and she was in the earlier version of the story.) Also, if you see the “Your Friend the Rat” short, when they talk about how many descendants one pair of rats can produce - It looks like the “parent” rats are meant to be Django and Desiree.
I always assumed she was an inquisitive relative.
Bigger question is why does she have a ‘Neeuuu Yoork Ach-sent’? 
Seems to me like she’s just a customer at the restaurant. I always thought that Remy had told this story to anybody who asked him lots of times before, and this was just a normal thing for him.
I guess you’re right. Besides, I would hope his close family would already know his life story.
I refer to that rat as disinterested rat. After the whole story is told, the only thing she can muster up is “How do you know” Really?
A teen rat sounds plausible. Perhaps its one of his cousins? Its a large clan, he must have a lot of distant cousins. Obviously that rat is related somehow, since this is all one big family.
I think she was just an unimportant character that was just there to bring us to the end of the movie she could be a relative or a rat from the street the world will never know!