This should be the need of the plot. As you said, the initial task of Youlian is not to stay on the earth for a long time. She first came to warn Eddie that King Galtan was going to assassinate him to get back at him for invading the country of light. You Lian was chased by King Galtan and forced to land on the earth. After being rescued by Meng Yan, she temporarily lost her memory. In order to help her recover, Huimei of UGM took care of her and named her star Liang Zi.