Sunday’s backstory is Quite Sad, as she as a Collective, feels like the only person who Truly Appreciates her, is the one who turned her into a Monster. The Day Twins Themselves were Abandoned as 4 year olds, and left to Rot, Slowly dying away until The Scientist found them. Danielle was stolen away by a mysterious Stranger, along with her Siblings Mettablook and Madaton. Danielle found herself as the Primary Vessel of the Amalgamate. The Day Twins are 6 by the way. Sunny, like Flowey, started off as a child. However, she was a Stillborn baby, and turned to dust on a Marigold, which later came to life as Sunny.