Indeed, she’s going to be someone you haven’t seen in the anime or the manga.
On Gematsu, we read that Arc System Works’ fighting game, which has a closed beta running at the moment, will get new characters. Yamcha and Tien need no introduction, if you are knowledged in the world of Dragon Ball, you know about them. However, Android 21 (C21) is a newcomer, who was designed by Akira Toriyama (he created Dragon Ball!) – she’s going to be a central character in the plot, and her intelligence will be comparable to Dr. Gero’s, the creator of the Androids. She looks a little mysterious going by the first images.
Arc System Works’ game will be published by Bandai Namco in February 2018 on PlayStation 4, Xbox One, and PC. Until then, have a video of the closed beta: