Phoebe is the daughter of Sally and Ken and the granddaughter of Bowser Junior and Joseph. She was born in Bowser Junior's Playtime 4 and died in Bowser Junior's Playtime 5. She was dead after Junior shove her in in the microwave after she wouldn't stop crying. Phoebe is mistakenly called Sebastian in the beginning due to Sebastian being a boy's name and the baby being a girl.
Bowser Junior started shaking Phoebe in a rage and Cody and Joseph started screaming in fear for him to put the baby down. Toad came out with a gun and attempted to shoot Junior.
In Bowser Junior's Playtime 5, in order to make Phoebe quiet, Junior put her in a microwave and turned it on. Sally later discovers this and the rest of the family gets mad at Junior.