How does your naughty naughty Imp get to the material plane? Probably something evil... and naughty...
Kulzu is a bit of a rarity among Devils. She's an Imp who actually likes her current existence as an Imp, and who has no desire to be promoted into a more powerful form. She much prefers her tiny size, her small shapeshifting abilities, and her innately poisonous nature, and likes that being a weak little Imp keeps her on the Material Plane doing other devils' dirty work and far, far away from the horrific violence of the Blood War.
At the moment, if all things go well, she'll be working for ExisD's Erinyes character as a sort of general lackey, while also running her own projects on the side. Mostly assassination work and illegal poison sales, but if ulthakptah allows it, she'll also have special dispensation to offer up Faustian Pacts (Fiendish Codex II) to help with her own soul-harvesting work.
Naturally, working for a Devil sent to corrupt souls in the Material Plane, she arrived by getting a Plane Shift with her boss, or however else Devils get to the Material Plane.