Yukina is Tsukumo's younger sister, and is horrified to find out that her older brother has fallen in love with an earth woman (Minato). She promptly leaves for the Nadesico to assassinate the woman in question, but after meeting her, she realizes Minato is a very nice woman and understands why her brother fell in love with her.
When her brother is murdered, Minato becomes her only family. Within the next few years, Minato becomes like a mother to Yukina. She often bullies Jun, but it seems because she has crush on him. (Source: Wikipedia)
