Born a member of the Red Tribe, Ultrawoman Lily later in life decided to change her color to yellow as part of the 'Alt-Color' Culture of her generation, which felt that Ultras shouldn't be distinguished by their colors/ethnicities or fall into traditional tribal roles. Since the Ultras had grown out of most inter-tribal racism ages ago very few people except the staunchest traditionalists saw a problem with this, and more than a few seniors decided to add, subtract, or change colors as well.
Lily grew up on a family farm outside Crystal Town growing Silver Grass and raising kaiju to be used as sparring partners for aspiring warriors. Unlike most young Ultras, who dreamed of joining the Inter Galactic Defense Force or Civilization Guardians or any of the other Ultra groups fighting monsters out in space, she figured that all sounded Ultra-dangerous and she'd probably be happier staying on the farm - though she conceded the monster wrangling she'd spent her life doing on the ranch would translate well to kaiju fighting - when some of her friends dragged her to a recruitment rally with Ultrawoman Beth as spokesgiant, and she was so inspired she decided to join the force!
Also she realized she was gay, which everyone but her had already figured out.
And so at the tender age of 4729, she graduated from Ultra School along with her roommate and childhood friend Iris whom she practiced kissing with for purely platonic reasons, though several of her instructors observed her definition of justice was rather 'rigid,' 'inflexible,' 'simplistic,' and 'overzealous.' She also seemed to have a thing for older women and had a praise kink given how she reacted to some of her teachers congratulating her on getting a good grade. It was decided to assign her to a relatively quiet sector of space lacking in planets with the sorts of resources that would make them targets for invasion where she wouldn't be forced to make many complex decisions beyond "Do I punch monster? Yes/no."
Unfortunately, Kyukogo happened to decide to conquer one of the planets in that sector for the only really valuable resource that mattered: its people, prompting Lily to get involved, despite the treaty between the Land of Light and the Hegemony and her superiors explicitly telling her to stay out of it. The rest is history.