I always thought that having a character that is both a traveling companion and a rival won't mix well together, because they would most likely get exposed to each other battle styles very often, hence ruining their surprises. There's a reason why Clemont temporary left the cast before his gym battle against Ash in XY. As for the pokegirl, I'll probably go with the female protag from the games, with a Scottish accent.
But that does provide quite a challenge for both the Trainers and Pokémon. Yes, you know what your rival can do and will likely do, but they also know your moves and tactics as well, so how do you adapt to that? And also, there is a lot of storyline potential with Marnie being both a companion and a rival, maybe best starting out as a rival but becoming a companion later on, especially if the rumors of her wanting to change the League's sponsorship system turns out to be true.