To give some more background here:
Chubs' real name is Mandy, and she had a sister named Muffin (same litter) that was put down six years ago due to arthritis. To be honest, I was quite uncomfortable putting Muffin down since she seemed to be perfectly fine at the time (plus the fact that I only found out while we were at the vet), but with Chubs, it was her time. She only recently started going from being her usual self to ill within the past two weeks or so. She earned the nickname back when I "adopted" her in late 2011 by moving into my grandmother's part of the house (who passed away from liver failure a few months prior). At the time, she was heavily, heavily overweight. At least 25 lbs, if not more.
By changing her feeding regimen to once a day (instead of my grandmother's "whenever she meowed" approach), we managed to get her back down to a healthy weight, but the nickname stuck. Fast forward ahead a year or two (which would be around October of last year), and my fiancee (better known here as Witchscat) moved into the same space. Within a month or so, my fiancee quit her job (under all of our guidance -- they treated and paid her like total garbage. Most of them anyways. She deserves a ton better), and from that point on, she would spend almost all of her day upstairs with Chubs.
My Mom insisted I get her to mingle more and get more comfortable with the entire house, but I saw no need, so I never addressed the issue. Looking back, I'm VERY glad that's the approach I took, since it allowed my fiancee to spend that much more time with Chubs prior to her passing. It wasn't until about a month ago that my sister's family moved out. That's when my fiancee started coming downstairs to play video games, go on her iPad, etc. during the day to keep herself occupied. But at night, and in the morning, she would be upstairs, spending time with Chubs.
My fiancee shared a very, very special bond with Chubs, as stated. We have three other cats currently (one of which is probably going to move out soon since it's my sister's cat), one aptly nicknamed "wumpus", one named Molly, and one named Sammy. Sammy returned from the vet at about noon today (I brought him home), and while he's the most social of the bunch, my fiancee didn't spend nearly as much time with him as she does with Chubs. That's why it's so hard on her right now.
If there's any good news in all of this, it's that The usual life expectancy of our cats means we won't need to face this issue again for a few years. Only one of the "oldest generation" of cats remains (wumpus, which we aren't terribly fond of anyways)., the rest are under 10 years old, so they have quite a ways to go.
Thank you everyone for the condolences. I'm going to miss her, but my fiancee needs far more support than I do right now.