on the Japanese "how to play" video, around 4 minutes and 21 seconds, it shows the sticker book. As we know now, in the SSE, you can power up your characters by placing stickers on the bottom of their trophy. around 4:21, a sticker of groudon is highlighted, and it shows what characters can use it in the SSE adventure mode. You can clearly see Jigglypuff and Lucario in the bottom right hand corner.in the next screen, a sticker from Mother series is highlighted, and you can see ness and lucas. This doesnt bode well for Mewtwo, as he is absent from the screen when the groudon sticker is highlighted.
The Japanese video