The last of the trio, minor spoilers in effect.

When we first meet Arven, he was kind of jerkish, he bemoaned seeing the Motorcycle dragon and ends up giving the legendaries to you, he would later recruit you to take down Titan Pokemon to obtain Herba Mystica, special herbs that can create Titan Pokemon on such a big diet but it can also help restore your Dragon's powers and heal what the Pokemon Center couldn't.
Turns out Arven has a Pokemon injured to the point of near death, his Mabostiff regaining his strength again through the herbs.
As the son of the professors, yes professors plural, I regard Turo and Sada as a couple despite the different titles, they worked tirelessly in Area Zero researching the past and future, the zone a great natural crater inhabited by strong Pokemon, think Cerulean Cave, poor Arven's attempt at reaching his parents failed and his Pokemon almost paid the price for it, his resentment against your dragon stems from it first appearing in the world and the professors abandoning Arven to keep it a secret, until the events of the game kickstart the story.
You can't blame him for his unsocial behaviour, parental neglect is very messed up and it doesn't get much better for Arven either, you'll see what I mean.