I don't mind Diantha but she gets like zero screentime, the first time you meet her she clashes directly with some of Lysandre's philosophy which is pretty nice but then you see her off hand one more time in Coumarine where she just says she wants to battle you someday and then never again until the end, whether it's just seeing her about more, or if she gets involved in the Flare plot, or whatever. Nothing.
Really XY's main problem with its characters is wasted potential, I actually find most of the characters likable in the first place, but they never develop or get any depth other than Lysandre and those involved in Looker Bureau, but mostly Emma there. The closest is Serena/Calem have some depth on an analytical level, and if you find Trevor's unmarked house in Lumiose you find out the whole reason he wanted to adventure was his parents abandoned him and his sister to be with their Pokemon instead, but they never do anything good with that like maybe have something where what he really wants to do is eventually leave the region and search the world for his parents, or even just spend time showing he wants to try to understand why they would abandon their children in his own way by going on an adventure with Pokemon himself.
Really the handling of Trevor bothers me the most of all the characters. You can't even lose the "Pokedex Battles" with him after the first.
