She was born onto Moo Moo Farm with other Miltank. She had known her mother, but often didn't get to see her. She grew up with other Miltank, loving and defending the younger ones. They had called her mama, as they had little access to the parents most times too. Sunshine tried to constantly visit her mom, but she could find no way to find her. Becoming more lonely, the Miltank became very independent from a young age, and she felt alone except when she was with her younger friends. The owners of the farm gave her a regular blue ribbon to distinguish her from other Miltank - all the other ones got different ribbons too - and she was forced to keep it on, but she didn't mind as she thought it suited her. Especially the fact that they had tied it into a bow - she thought that made it look even prettier.
As soon as she became of age, the farmers tried to get milk from her. She didn't remember much of the traumatic experience except there were hands going for her stomach and touching her in a way she did NOT even remotely feel comfortable with. She kicked and slammed and broke the restraints they had on her and ran free. She didn't get near them when they approached her from then on. Since she was of age now, they put her into the adult stalls, and she got to see her mother more and actually develop a bond with her. And she felt a little more defenseless in that time than she did in her entire life, because she finally didn't have to be 'mother' to a bunch of other Miltank - even though she had loved to. She lived 5 more of her years on the ranch before she decided to go out on her own when she became 20 in her years. Her mother was disheartened by her departure, wanting to get to know her daughter even better. Sunshine offered her mom to go with her, but she said she couldn't because she was too old and starting to get feeble; she would need people to take care of her. Sunshine managed to escape in the middle of the night, not letting sadness and homesickness tempt her back.
In the next few weeks of travel, she felt completely alone and out of place. Even endangered. Most Pokemon she crossed paths with would simply stare, out of confusion but she couldn't tell the difference. Until she came upon a Snubbull, friendly enough to talk to her - out of lack of fear or intrigue, it wasn't clear which.
"You look really out of place." she giggled. Sunshine was so recluse at that point, that her lack of socialization left her yearning to talk to this little pup. She scratched her ear in contemplation
"How am ah outta place, pumpkin? Ain't there othe' Miltank 'n these here parts? Ah mean... I 'avn't seen any o' my kin' 'ere, shu-ah, but.... "The Snubbull shook her head, still smiling a little.
"Nope. You're as out of place here as a Charmander in the ocean."
This Snubble introduced herself as Reylyn, and they became immediate friends. Eventually, and after the Miltank explained she was on her own, she told her that she was a runaway too - or an orphan - and most of ther other friends she had made in this forest were too. They became a family, and for the next 5 years of her life, she lived with them there. She was much happier, and forest life provided a lot more things that she could do to live comfortably.
The infection came and started sweeping over the forest, and the scare instilled in all of the younger ones and Sunshine. Eventually one of the youngest ones contracted the illness, and in agony but out of a strong sense of dignity and love she managed to kill them with her own bare hands. She fell into a horrible depression, and became hopeless and borderline suicidal out of her own anguish. She was still their mother figure, but she didn't want to have to deal with killing them if they were hopelessly infected. It wrenched her heart.
One night when all of them were sleeping in their sleep space (an outcropping in the forest space) a rouge undead came and attacked them. Before she could properly retaliate, it killed all the others before she could land a hit. Sunshine was a high enough level to attack and eventually kill it once it went after her, and it helped that she was fueled by rage. Horribly distraught, she ran from the forest and towards the farm, deathly worried about her mother's safety. When she got there, she found the farm was abandoned, and somewhat destroyed. She cried in agony, and went on her way after staying at the farm until the dawn.
A week or so later of traveling to the east, she found a Pokeball on a random route in Kanto - she wasn't sure where she was. After picking it up and realizing she had never seen something quite like it, she managed to press the white button on it and out popped the most menacing thing she had ever seen in her life. At first she was frozen still, until the Tyrantrum leveled with her and thanked her profusely for letting him free. He was almost crying, it proved pretty endearing to her despite his size and terrifying looks. They bonded quickly, and started traveling together, running from hoards and trying to find other living Pokemon.