Rupp caught fire with the Rangers AAA team, hitting .274/.346/.540 albeit with a 34.6 percent strikeout rate. Then the Rangers let him go, where Rupp would go on to struggle with the Twins and Mariners. Overall, it was a pretty forgettable and frustrating season for him.
Defensively Rupp is not known for his framing skills. Really, he's not well regarded as a backstop overall. However, he has power, and he could work with Posey on some things.
Projection system Steamer doesn't like him at all in 2019. It projects a .202/.275/.350 batting line with no playing time. Rupp should display more raw power than that. Baseball reference lists his Marcel projection, which is .233/.302/.414 in 233 plate appearances. That seems like a more likely outcome.
Currently, the Giants have Aramis Garcia as their backup catcher. He's a better defensive catcher, but offers nowhere near Rupp's power potential. It will be interesting to see if Rupp displaces Garcia when spring training is out in 2019.