Party popper is an attention seeker, her older sister was more emotional, and often bullied so poppers parents had to keep a close eye on jet puff, making popper feel left out. It only got worse when puff found her calling as a firefighter, and becoming a hero to ponies she saves.
once popper hit her teens she started acting out, getting Really rowdy and into fights, falling into the wrong crowd coming home with piercings and other such things. Her and rainbow dash started arguing a Lot, their relationship getting strained, while cheese couldn't do anything but watch them spiral.
Popper and jet puff don't get along much to puffs dismay, puff wants them to be close like they were when they were kids but poppers resentment for being in puffs shadow is keeping them from reconciling.
however popper Does love her sister and knows deep down it isn't puffs fault, so whenever puff gets bullied the next day both popper and the bullies are sporting some bruises.
Popper is Loud, and Crass, trying to get everyone's attention on her even if its bad. She has a bad temper and is quick to throw the first punch. If you can manage to get past her surly tough mask then you've got a friend for life.