At least, he thinks he is the strongest Rival. Since they'll have different Monsters, it really depends on your own Team who the strongest Rival is.
Her he is:
Noone really knows why he and Higashi became such fierce rivals, maybe because there aren't any other boys their age in Hajimachi, but seriously, they could also have become friends. He insults Higashi whenever he can, and there aren't many people that get respect from him. He doesn't really like Minami either. Strangely enough, he never really insulted Kita, but actually, they barely talked at all. He just seems to ignore her for some reason, maybe because she is so silent and gives him absolutely no ground for insults?
Of the four Main Characters, he is the oldest. And that just makes him even cockier. But where does that cockiness come from? Again, noone really knows.
His mother, Rubi, is very gentle and cares much for her children. She handles Nishis cockiness carefully, but for her it's easy, because Nishi shows her great respect. She is very strongwilled, and thats good, having three boys, one girl and a husband that rarely is home.
His older brother, Aio (this name actually derives from a gemstone too, it is the formula for the sapphire (Al2O3), minus the numbers) works abroad as the assistant of Professor.
His older sister, Crystal, is not a Leader. She works as a journalist, as one that is right at the scene of an event, asking people questions, taking photos and such.
His younger brother, Kohaku, is a nestling. Nishi mostly ignores him because his mother wouldn't like it if he insults him. He loves his siblings, and they love him, except Nishi. He rarely leaves the house, and when he does, only with his mother.
So, as a special, here are our thoughts on Nishi:
Rief: "Well, i think he is the perfect rival for Higashi (and thats good, since i created those two). You could say they are complete opposites. And i think girls will love him, characters like his are somehow popular. By the way, his sunglasses aren't just an accessoire, it has a meaning for him that he wears them... He wears them so he won't have to see all those people inferior to him, he fears that they might make his eyes bad since they are so stupid.
Well, i hope you will like him, as all the other characters, and who knows what will happen to him on his journey?"
Mobychan will follow, since she is asleep right now.
Mobychan: "I think Nishi's a nice change when compared to the rather "normal" Higashi. His character is kinda extreme, but I hope we will be able to show his development throughout the story as well as we think. Would be kinda interessting to get to know how many girl will play as Higashi just to have him as rival..."
Draeron:"Out of the 4 main characters I like Nishi the most."