We can't expect Tierno and Trevor to be Serena/Calem or other main antagonists. Every other game had focused on one, while X/Y provided 4. It is pretty much sufficient for me as long as they made enough appearance and set their own goals. The games did not provide structure for them to develop further, as in the lack of Pokémon Contest Spectacular of Showcases for Tierno for example.Huh, character development is what I felt Tierno and Trevor lacked. Just about all their dialogue was about dancing/catching and without any reflection at that. To me Trevor and Tierno felt like they were on the exact same page at the end of the story as they were at the start.
I find it interesting that you think Brendan/May didn't have any development though. Because they did go from motivated new trainers that eventually figured out they weren't that good at battling and decided to follow their father's footsteps instead.
Brendan and May were there to follow their father's footsteps from the beginning. Professor Brich is known to be studying Pokémon on field and gave one to his children to research as many Pokémon as for him during their journey. The title "Trainer" is lacked for them. They didn't challange Gym leaders as far as I know, until confessing why they are no longer interested in battling. Interestingly enough, they had Mega Evolutions at the end of the game, unless they played Sycamore 2.0 there in terms of research like in anime. Can't say the same for Wally, however. In OR/AS, only rival, except Lisia and Contest Spectacular, was him. It was good to witness the little, sick, kid to catch his own Pokémon, develop the bond with it, travel and even made his appearance in BuzzNav while sweeping Gyms, post-game adventure and so on.