When the short alchemist and her competition for the throne were eaten, she decided to follow Alphonse and Gluttony since they would probably lead her to where the Stones were being made. At Father’s place, she was only observing from the ceiling when Ling and co were transported back.
After Ling turned into the new Greed and the alchemist left, she was full with envy and rage. How dared that bratty and spoiled prince rob her prize? She deserved it more than him, she had come all this way just looking for a way to keep her family alive and honored and he comes and steals all her chances of living.
Father sensed Sun in the room and turned the ceiling into a smooth surface, so she couldn’t hold on to it anymore. Falling on the ground and breaking some bones in the process, she healed herself quietly with Alkahestry and decided to act like she had died from the fall. Father noticed the trick and said that she could make a good Homunculi if she wanted. She kept acting like she was dead, the opportunity was too good. He was probably trying to deceive her and then kill her.
Father told Envy to put the Philosopher’s stone on her body to see what would happen. When she felt the sting of the needle, her world melted. Inside her mind, an Homunculi appeared. “Oh look, a nasty human. Am I supposed to take over your mind young girl?” She screamed in fear and tried to make the being disappear. “Ooh, I got a fighter.
Interesting. Perhaps, we could join forces dear. I’m Youth, the thing you seeked for” Sun didn’t know how the thing knew that she was looking for the Stone and was afraid of it. She didn’t want to fight with Youth, let alone against her so she did the most reckless thing she had even done.
Sun broke her own mind into pieces, leaving her emotions a wreck and her conscience unable to work. Her sanity faded and she could no longer see what was happening in the world outside. After some time, she went numb. She couldn’t feel, see or hear a thing and the nothingness in her broken mind was driving her crazy. Youth was trying to bring her back, she wanted the control over such a healthy and powerful body.
After fifteen years, Youth was able to fix Sun’s mind. But when that happened, Youth was part of it. Both the human and the Homunculi shared the mind and the body and now Sun couldn’t do anything about it, it was too late to separate one from the other. Sun’s anger was huge, but she was too weak after all that time with no feelings. Youth took control of her mind and made Sun back to her old healthy state again.
After that, Sun grew to hate the blasted country. She wanted it destroyed. Both consciences agreed on that, seeing Amestris fall on their hands. Youth was glad that Sun finally accepted her and both of them acted together in harmony. Their plan and mission was to make the country fall on their feet, every single human obeying to their will and destroy those annoying Militaries.
Sun always had long black hair, so straight and beautiful other princessess would always be jealous. She loves her hair and spends more time taking care of it then a proper warrior should. Her skin has an alabastar colour, so pale some people once thought she was dead when she fell asleep. Her used to be a deep brown colour, the only thing she hated about herself. When Youth got into her system, Sun was glad to notice her eyes had turned into a crimson shade that looked great with her black hair. Her Ouroboros tatoo is located on her belly button, so she never shows it to anyone
Though she still keeps her 13 year old appearence she is already 28 years old. Youth didn't let her age so she'll keep this appearence probably forever. Though she doesn't need to eat and the Stone always healed her and kept her alive, after 15 years she was as thin as an anorexic girl. Youth helped her gaining weight so now she's healthier, though still very thin.
Sun is a calculating person, cold and vain. Even though she never related well to people she always loved her family and because of that, she traveled to Amestris. She isn't amazed easily and is power hungry. When she heard that she could inherit the throne, she gave her all to obtain it. When her hate is awaken, it doesn't disappear and right now, it has only increased over time. After breaking her mind, it became weak and her sanity wasn't recovered fully, so she can sometimes have break-downs.
Youth is very different from Sun. She was made out of Father's youth and childish mentality so she can be the less rational part of the two of them. Still, she is quite clever and smart. She also has a great disdain for humans, thinking them frail, predictable and generally stupid like some of her fellow Homunculi and she takes great pride on being better than humans. She took a liking on Sun and her body, the only reason she actually tried to fix Sun's mind, but has a small abandonment issue over the fact that Father didn't even try to fix her.