I get that a lot of people hate Arven because he's so spiteful towards Koraidon/Miraidon, but that's only really fair to say in the early game. The end of the Path Of Legends really sheds a light onto why he might be so bitchy towards the Pokemon that makes him sympathetic.

If you remember from the end of the Path of Legends, he explained that his parents (Professor Sada & Toro) are deadbeats who favored their research over raising him. The legendary was in their possession before, and now it's in yours. The professor puts a lot of energy into that Pokemon. Arven is most likely as spiteful as he is because his parents care more about that fuckin dog dragon motorcycle thing than they ever cared about him.

Sure, he was outrageously bitchy at the start of the game, but remember this is a 18-20(??) year old man who was neglected in favor of Pokemon now having to watch you care for one of the Pokemon that he was neglected in favor of. That's kinda fucked up, Game Freak.