Teana was the one who took out Nove in StrikerS along with Wendi and the one with the twin swords. Subaru was busy taking down the brainwashed Ginga with her friendship punch.

They are all cyborgs from the same string of research, and the "good" ones were probably going to be taken in by Ginga and Subaru's father during the final scenes, so it isn't all that surprising that she views her as a "sister" now, particularly because they are so similar.

I'm surprised how quickly they revealed Einhart, much less that she will probably become quite friendly with Vivio.