Rainbow Raspberry is a one-time villain from the 2016 reboot of the Cartoon Network animated series The Powerpuff Girls, only appearing in the episode "Checkin' Out". She is a cereal mascot, who Bubbles imagined as a friend, who turned into an enemy, in a fit of petty jealousy.
Bubbles went to the grocery store to buy some cereal. Blossom told her that she could only choose one cereal and it was very important that she chose the right one. While looking through all the options, Bubbles had a lot of trouble making the right decision. It also did not help that the fate of the world rested in her hands, what with the Hungry Alien planning to destroy them if he did not get his delicious dinner.
While pondering which cereal box to choose, Bubbles started imagining all of the cereal mascots coming to life. One of the cereal mascots was Rainbow Raspberry of the eponymous brand of cereal. Rainbow Raspberry lept out of the box and happily asked Bubbles to pick her. Bubbles had lots of fun, making her decision, once she began partying with all of the cereal mascots and having a great time, making friends with them.
Eventually, after enough partying, Bubbles had to make a decision and it was not easy, given how much the other cereal mascots were pressuring her to do so. "Pick me! Pick me!", they all said, to no avail. Bubbles still could not make up her mind. "Choose all of us!" they offered, but Bubbles sadly told them that Blossom only allowed her to choose one. Every cereal mascot went right back to begging "Pick me! Pick me!".
Bubbles finally chose Cherry Chimpanzee, which angered every other mascot. In an awkward and instant shift of tone, every mascot turned on her and chastised her for not picking them. Rainbow Raspberry tried to guilt trip Bubbles, by telling her how long she's been sitting on the shelf, just rotting away. She feared that someday, she would expire and nobody would get her.
Rainbow Raspberry, along with all the other cereal mascots cornered Bubbles and prepared to attack her but Cherry Chimpanzee showed Bubbles a hose, that sprayed out milk. Bubbles used the milk hose to blast all of the cereal mascots. Upon being squirted with milk, all of the cereal mascots melted to oblivion.
However, this was all happening in Bubbles' imagination. In real life, she just took a bunch of milk and started dumping it all over the cereal boxes, which led to her getting promptly kicked out, without any cereal at all.