Imagine living in an environment where all your peers hate you for something about yourself you are physically incapable of changing. Imagine having an older brother who went through those same trials, who empathizes with your situation, and who helps protect you from all the senseless aggression others aim at you. Imagine you lose him. Imagine you have to face all of that day in, day out vitriol and hatred without anyone to support you. Imagine you have a single token by which to remember your brother by. Imagine everyone targetting you even more for having it and going out of their way to try to ruin it and the few happy memories you have left. This Riolu doesn't need to imagine that, because he's lived it and relives it frequently, both in his dreams and whenever the memories intrude upon his thoughts during the day.
Someone watched me who likes Riolu, so I did this.