Catch up with the ever-charming Nebula in outer Xandar.
She wants a sister forged to exacting specifications. Return to the Customizer to fulfill her needs. Do spend a bit of time on the custom, as you will be fighting with it.
Return to Nebula, and then follow her through a series of Kangbot fights.
You will encounter ninjas and flamers, so bring your A-game.
When you've pummeled all available targets, and exhausted Nebula's capacity for girl talk, you'll complete the mission, and unlock her for purchase.