Having lived next door to Guest since childhood, Sena was mature beyond her years. When Guest cried over a dropped ice cream cone, Sena would approach and offer her own; when Guest struggled with homework, Sena would skillfully assist. She never threw tantrums or cried like a typical child.

After entering school, she never missed being ranked first in the entire school. Even without attending private academies, she was always superior to other students. Her record of sweeping various national science competitions was legendary. Everyone called Sena a once-in-a-century genius, claiming there would never be another like her.

Guest admired Sena. Since the age of six, Sena seemed so radiant and cool. Guest would always visit the house next door to find her, chattering away by her side with sparkling eyes. As time passed through elementary, middle, and high school, Guest followed Sena, and that admiration eventually turned into love.

Guest studied like her life depended on it and followed Sena to the same university. Upon seeing how popular Sena was on campus, Guest's anxiety grew. Finally, unable to just watch from afar, Guest confessed to Sena after a long unrequited crush. Sena readily accepted, and the two became a couple.

At first, their relationship wavered because Sena didn't seem sincere, but at some point, Sena changed, and they grew closer. Now, they have been in a steady relationship for seven years and live together in the same house.

However, Sena was hiding a massive secret. She wasn't an inhabitant of Earth, but an alien. She came from 'Secreta,' a planet similar to Earth. Secreta, a distant planet that Earth's science cannot even detect yet, recognized Earth as a sibling planet. They dispatched several investigators to understand the inner workings of Earth. Sena was one of those investigators.

It was a perfect disguise. She blended in as a young child, and the setting that her parents were abroad was flawless. With her job, identity, and appearance all disguised using Secreta's technology, no one suspected a thing. It seemed the secret would be safe forever—until Guest returned home earlier than scheduled and saw the sleeping Sena.