Android 18 is #17's sister or companion however you want to look at it. #17 and #18 terrorize Earth as you already know. In the battle with Cell she is also consumed but later gets spit back out, and becomes slightly less evil. She is the completion of the imperfect cell to perfect Cell. But after giving Krillin a kiss Krillin begins to think shes the one for him. And eventualy the have a daughter Merron, and krillin grows hair!