>>52334146>he didn't play the gameArven didn't know there were two legendaries.
The one he knows is the one he grew up with, which was actually aggressive and attacked a pokemon in front of people, which caused his mom/dad to have to confine it to Area Zero and monitor it there. Arven sees Miraidon/Koraidon as what brought his parent away from him because it was aggressive and out of control.
It's totally reasonable for him to be weary of it and to spite it in some way, eventually he even warms up to it, making extra sandwiches even if he's tsun tsun about it.
When he tells your legendary that it ruined his life, he still didn't know it wasn't the same legendary, it's AI Sada/Turo that explains that to you.
