Mimi the Guinea Pig

Poor Mimi. She leads a good life and is a good friend to Tibby, but when she dies, she winds up tossed in the back of the freezer. Bummer, right? 

Mimi symbolizes the life cycle. You wake up, eat a little bit, take a nap, maybe play with a ball, eat some more, and go back to sleep—or something like that, anyway. Mimi lives the easy life, with "no decisions, no disappointments" (2.2), and she enjoys her time on earth, but then eventually dies. It may not be a terribly adventurous circle, but Mimi definitely represents the circle of life in this book. 

That's not all, though, and though Mimi's tiny she brings a whole lot of symbolic meaning. Tibby puts Mimi in the freezer when she dies for a good reason. Cryonics? Nope—avoidance. See, when Mimi dies, Bailey's dying too—and just like Tibby stops taking Bailey's calls, she also shoves Mimi out of sight. Whether it's her guinea pig or her friend, Tibby does her darndest to avoid facing the fact that she's losing both of them. Mimi symbolizes this fear of looking loss head-on that Tibby has to nagivate. 

And navigate it Tibby does. Eventually she visits Bailey and has a quality goodbye with her friend; and after Bailey dies, Tibby finds the courage to lay both of her loved ones to rest, and buries Mimi alongside Bailey in the cemetery. When she finally buries her frozen furry friend, we know that Tibby is finally at peace with the losses she's experienced, and ready to get on with living.