To avoid veering off-topic, I'm posting here - at present, I like the idea that Shauna and Calem/Serena will be the rivals (Shauna takes the one weak to you, C/S takes the one that's strong against you). There just doesn't seem to be as much information about Shauna's personality as there is about Tierno's and Trevor's; she's described as being eager to "discover her dreams" which I assume she'll do by going on a journey and battling, like most Trainers. In contrast, Tierno is pre-occupied with dancing and Trevor just wants to complete the Pokédex. And, while this is just my own speculation, I think it's possible that Trevor and Tierno may already have their own Pokémon, which would explain why they were entrusted to distribute the starters and Pokédexes.