If you are starting her (and the NPC) off at first level and are using an urban setting, I suggest a street smarts sort of deal where she can pull small jobs, get into scrapes with guards and other NPCs, and she can get used to movement, combat and using her feats/skills. At low to middle levels, you can introduce a mysterious benefactor who hires them to find artifacts, steal art, plant evidence, kidnap or waylay people, etc. At mid to high levels, the thief has come into her own and has to negotiate the political environment of the thieves' guild and the town politicians.