Qiwi Lin Lisolet starts the journey to the Spider system as a teenager, the daughter of one of the senior captians of the Qeng Ho fleet. She's a tomboy, brilliant and always underfoot, causing trouble and sticking her nose where it doesn't belong. Think a space age Arya Stark.
After the battle between the Qeng Ho and the Emergents, she becomes a crucial link between Thomas Nau, the Emergent leader, and the Qeng Ho survivors, who Nau needs to complete the mission. The charismatic Nau manipulates and seduces Qiwi (grossssssssssss, it's awful seriously).
We need someone attractive, spunky, sympathetic, and who can play a pretty wide range of ages, from say 16 to mid-late twenties. So, I'll pick
Let's see what some of the other cast members think about this:
Book: Deepness in the Sky
Director: James Cameron
Pham Nuwen: Bryan Cranston
Score: Aaron Copland
Thomas Nau: 40ish Patrick Steward
Qiwi Lin Lisolet: 19-year-old Kiera Knightley