Android 21 being a spy for the Saiyans feels so random. Nothing about her character really has anything to do with Saiyans; she's a Majin Bio-Android with just a tiny bit of Saiyan cells due to having Cell's DNA. It would have made more sense if she was a spy for other Majins (or characters associated with Majins, like Bibidi, Babidi, Dabura, Yakon, Pui Pui, Spopovich, Yamu, etc) or androids associated with the Red Ribbon Army.
@greengrasstree: Seriously? I mean I get what you’re saying, but this is a story where Krillin kicks Goku (or rather, Kakarot) and Vegeta’s asses with ease. This is the kind of crossover where multiple characters from one series are placed in another series (usually replacing the characters with similar roles) and made to experience the series’ events, or similar. I’ve seen it done before. There are bound to be some differences put in by the author/artist here and there.