Gren: She's not a ninja, she's an Agent!
http://elderscrolls.wikia.com/wiki/Agent

Higuera: nope,knee-arrow sniper badass

Bob: @Gren:

Practically the same thing. Both master in sneaking and stealth and even shown to have mystical powers. (Though I may be stretching the truth a bit here).

Mediocre_Scrublord: im pretty sure that the idea of the agent is that it avoids combat

Gren: @Mediocre_Scrublord: Well, I think ninjas were meant to avoid combat as well. But I think ninjas are more related to Assassins mainly due to their goals (to kill people).
While Agents are more like Spies. They are the sneaky charming info-collectors saboteurs.