Natasha - a "combat - Slytherin". You don't want this girl after you. Her ambition isn't oriented towards achieving status, it's focused on getting the job done, so whatever her goal is, she will reach it. She is also, determined, subtle, she comes prepared (that's basically, what Natasha is all about), she fits the goal justifies the means mentality. She's pretty ruthless when she needs to be, and she can separate herself from the things she does - in fact, she can separate her different selves, she can operate under entire identities which are temporary, while keeping the real self somewhere inside. She is loyal, but her loyalty is selective (and that's also a typical trait of the house). When Natasha takes one for the team (but not really for the team, it's more for Clint), it hurts. I think that the loyalty she feels toward her partner (romantic or not) expands to the team during the film, so both of these feelings are the motivation for her decision to go into a war; that's why she becomes a soldier. Slytherins are usually more loyal to people than to ideas, we really don't weep over regimes falling, but they (we! I heart my house!) we'd do anything to keep our loved ones safe, and it usually means more to us than keeping the world safe.

(also? Slytherin myself. *nods*)