When Riley was born, there were her 6 feelings…
Joy, Anger, Disgust, Sadness, Fear and Killjoy…
Killjoy was the one whose purpose was to deal with bullies in school, tantrums or annoying persons without Anger…
She was also Joy's doppelganger and Riley's seriously freaking underrated feeling which reputation causes her to make a whole burnout…
After being rejected, outsmarted and humiliated by other feelings, Killjoy sworn to take control of Riley…
She killed Riley's parents once but discovered Bing Bong tried to takeover Riley's mind, so she pressed the babe-button to reset Riley's life and control Riley with her new accomplice Party Pooper (who was Meg's friend's KillJoy)
Realizing she went too far, KillJoy tried to do her best to let Joy control the console,
But one day,
When they were 9,
Riley and Meg were playing hockey in Meg's Garden,
Riley accidentally sent the puck in the neighbor's garden,
Meg went to search for it.
Unfortunately for Meg, her neighbors were a ill-tempered 70 years old couple who couldn't bear children playing near their house…
When Meg attempted to catch the puck in the neighbor's flower plant, the old woman hit Meg with her cane.
KillJoy, with a "get away from her" angry look, took control of Riley as she punches the neighbor and made her eat her lily-of-the-valley…
Some hours later, Carl Fredricksen found out his wife Ellie died in the hospital and in Riley's mind, FL managed to dispose of both KillJoy and Party Pooper by dumping them in the memory dump (something like Dip in "Who Framed Roger Rabbit" erasing memories and useless imaginary friends/boyfriends) …
But unbeknownst to him, KillJoy survived and under the guise of a Dream Production actress, she has sworn to avenge party pooper and make sure Riley gets friends with Meg again (since she protected her from Ellie) with no new bestie around Meg…