It seems like people love to trash these characters, but the truth is they're not your rivals, they're supposed to be just friendos. I honestly don't get what's wrong with them. They rarely appear and they act like most NPCs. To summarize them:
>Shauna likes cute pokemon and has a thing for the player, not interested in beating the league.
>Trevor wants to catch them all, he's shy and acts like a wise guy
>Tierno is a dance fag, he just battles so his pokemon can get better with their dance technique, which is why he uses "dance moves"