How exactly was Kotaro a wimp? As the show progressed, the more he dominated. He wasn't weaker, he was up against the big boys. This wasn't a cult of mutants, this was a military invasion. So the more he understood his own power and the more he trained, the more effective he got. That's why eventually Grand Lord Crisis had to bring Granzairus, the most powerful warrior, to earth. RX ended up becoming too much for what started out as an obstacle to him.