Tsubasa was technically an only child, but for a time, she had what may as well have been a twin.  The two had to fight dangerous creatures released after a ship crashed, but after defeating the last of the creatures, her twin Hikaru couldn't continue existing, and was no more.  That at least is what she thought, as did the Extra Terrestrial Law Officers that had then wiped everyone else's memories, and left with only Tsubasa remembering anything.