To make it short, I'd say the rival would be a character you really wouldn't want as your enemy, but you'd definitely want him as your good friend.
I felt he was kinda of a mix of two types of rivals we've seen before, which was either "your faithful and kind childhood friend" or the "mean and cold kid on the block." He can be capable of being very aggressive and violent, but at the same he can be your man to count on. As for his 'slogan' I don't really thinks he means to throw himself into a rage fit, but maybe he's trying to say something like "I'm getting fired/pumped up!". I can imagine him being hot-blooded guy, and perhaps even protective as he has a younger sister.
I think the sister factor kind of rules him out as being a complete jerk or anti-social guy like Silver, but then again we still know very little about him too. I'm sure he values his younger sibling, even going out of the way of walking with her until he met up with the protagonist. In fact, we even saw him tag along with our main playable character around the town. We don't know why he does, but it must mean he enjoys the player's company.
In the video we couldn't really see where the battle started, when, or how, but I can guess that it began once they've reached lookout place perhaps? Maybe the reason he followed you was because, kind of like tagging with Jun/Barry to Lake Verity, you were expected to take him to that place. Maybe a battle was arranged at the lookout beforehand? Probably. That or he suddenly thought to have the battle at the lookout because maybe he wanted to have the first battle at a 'memorable' place. I know it's just me thinking like that, but I kind of like that thought.
.... Of course I can be completely wrong lol!
Who knows, guess we have to wait, but I feel as though I rambled a bit too much.