From Les Miserables... Not! From Dragon Ball Z, duh!

Scenario 1: She casually strolls into your store when she and 17 are driving around trying to find Goku and kill him.

Scenario 2: You replace Krillin and you're living in Kame House after saving 18 from Cell. One morning you wake up to see 18 lounging around nude.

Scenario 3: Design your own.

This is not the homicidal 18 from the Future Trunks timeline but the main timeline's.