She tends to overeat and with that she also expands, this isn't even the biggest she can get but it is a good example of why she doesn't wear pants.
Flappy will eat almost anything and enjoys doing so, it mostly goes to her thighs leaving her a bit wide and she has a habit of underestimating how big she gets.
In addition to being a glutton she can be a bit of a pervert, Flappy will also resort to using one vice to satisfy another and is overall free-spirited in that way.

While not particularly attached to the sweater she wears she prefers long sleeved clothing as the sleeves are her arms, as well as a variety of other arm attached items (but no hands for some reason)
Even something like a long enough ribbon tied around her chest can be used as makeshift arms.
Lastly Flappy cannot talk or make any vocal sounds, she usually communicates with her sleeves which is where her name comes from.

Additional info: She's not actually a toon, she's a mimic. The same kind that usually become furniture or chests, however she ended up in unusual circumstances which led her to mimic a living creature.
(couldn't decide to keep the white spot or not...)