Most DC fans know the story of Supergirl, and within the realm of Injustice 2, the story is still pretty much the same, aside from the shift in timeline on Earth 2. Up until now, Kara Zor El has been a background player in the trailers without any real distinction of who she is to this universe, or why in the hell she wasn't a part of the first game since Superman was running things. Now the latest trailer gives some insight and depth to her story, and it appears she is one of the many heroes caught in the middle of the fighting. That's actually interesting and far better than the character deserves, rather than to be regulated as one of Superman's flunkies.
If they did this kind of trailer with all of the characters, it might entice more of the "DC Fanboy Nation" to just buy the game and play it, rather than spend their days complaining about how she looks and why she isn't a threat to anyone.