[Aries Walker]

As for Scarlett being in this movie, she's very popular right now, so I can see why they featured her so prominently. Also, Black Widow was very much needed, not only so (and here I go into spoilerland) . . .

Spoiler!

there was someone who could do the covert "other orders" actions on board the Lemurian Star, but also to provide balance to Captain America. He's getting his footing in a strange land, so he needs people he can identify with. Falcon is one, but even he doesn't also provide the contrast in personality she does; he is an honorable person who is having to live in a dishonorable world, while she is just the opposite. Both of their characters are strengthened from their relationship, and this is the logical place for her to be after Avengers.

Plus, she big-sisters him into asking out Sharon, and he (let's face it) kinds of needs to be big-sistered (or at least, big-siblinged).