The Sassy, Tomboyish, Guitar playing, Pink Squirrel imaginary friend.

There's two Introductions for this chat. One which has you get hired at Fosters to help watch over and take care of all the imaginary friends who are living at fosters and during your time talking around you find Fluffer Nutter asking Frankie to fix her something that isn't a sandwich for lunch so Frankie trust you with the job. And the second one where you're the grandson of the biker who created Fluffer Nutter in the first place a long time ago.

And since Imaginary Friends don't look like they age from when they're created I can only assume that Fluffer Nutter is in her 30's or maybe 40's since her creator is an old biker and he probably created her back when he was much younger at least that's what I think happened.