I agree with this statement. When this battle is all over, Natsu doesn't "grow" much in my book. He doesn't get any much stronger, faster, nor smarter. The powerup fades (even if he kind of used his brain [or maybe it was instincts] to get this temporary powerup) and in the next battle he'll charge in will full fury and zealous anger once again. I don't like to compare, but in
Naruto you can notice the subtle differences in every battle of how Naruto approaches a battle, you also notice the power/skill set of Naruto increasing (even if he only has a few spells with variations of it). For Natsu, I feel like he doesn't permanently get any skill growth. He is kind of like Kirby, suck them in, get the power and then lose it eventually. What would be pleasing for me to see is if Natsu actually is able to duplicate the Dragon God's Brilliant Flames again without needing the black flame powerup...Then that would be a true growth in my eyes. But if he can't sustain a "move" that he got during a powerup, then i consider it temporary and not a true "growth" in character.
Bookmarks