Banshee said:I'm not 100% sure on this, but I'm pretty confident that Tierno and Trevor aren't rivals, just friends. You don't actually battle either of them.
After watching thedecepticons livestream for quite a while (there were loads of users from this site also watching so I'm sure someone can back me up), The player, Shauna and Serena/Calem are the three of the group that receive a Kalos starter, and also a Kanto starter later on. Tierno and Trevor turn up late to the meeting with the professor and miss out. I know it doesn't mean that you definitely don't battle them, but even their character descriptions make it sound like you won't, and from all the time I watched they didn't battle you, or appear to have any Pokemon with them.
You are right. I didn't see them battling either. Only Serena/Calem and Shauna are your rivals. The rest are friends. Not saying they don't have own pokemon, but probably they won't challenge you. Maybe you and them will get chance to battle together in double and 3ple battles?