Bandai Namco are slowly teasing their way into getting Android 21 into the mix of Dragon Ball FighterZ. She is essentially a fantasy character made up for the purposes of the game in an alternate timeline. The redheaded scientist has (up until now) only been seen in stills showing off that she's part of the game, but not really showing off her capacity or what side she's going to be on once the game kicks into action.
For the time being, the trailer below would lead you to believe that Android 21 may actually be helping Goku and company, as the clip shows her attending to a knocked-out Android 18 with Krillin and Goku standing over them. In this scene, we get some dialog, but not much else as we're left to speculate.
What's more, we don't get any kind of info about her fighting style, which is a change of pace from how Bandai Namco has been promoting characters as they're announced for the game. The one big thing the character has going for her: she's not just someone's hopeful dream of the Androids, she is actually designed by Akira Toriyama, which means at some level her story for Dragon Ball FighterZ is going to have his touch.