There are some things about this story that I really enjoy. There are other things that I think could be made better.
First of all, I like the title. Next I really love the whole premise. Who better than Lulu to advocate the positive points of lightning magic to the terrified Rikku?
I really enjoy your use of onomatopoeia in the story. The way you use words like "BOOM" and "screeeeaaam." I even enjoy the use of all caps that you employ. It seems to suit Rikku to a T. She's so excessive all of the time that for a story of this length it can work. Even the overuse of exclamation points adds to Rikku's quirky attitude.
The problem is that when you add all of these things together your story starts to collapse under the weight. If you trimmed down a little on maybe some of the exclamation marks and caps, then the story would be a bit easier on the eyes for a start.
Now I'm going to weigh in again on more punctuation that bothers me a lot. I am aware it bothers me more than most, so feel free to disregard my comments if you like. It's the ellipsis [. . .]. Lots of people overuse it in fanfiction. You write dialogue quite well and so I think you could dispense with it almost entirely in this story. An ellipsis is really supposed to signal an omission from a sentence and it just visually breaks up the words on the page. It's distracting from the excellent content of your story in other words.
I feel the same way about the dashes in the story too. I think that if you got rid of the dashes and the ellipses you could keep the caps and exclamation marks and italics.
I love your idea about red eyes being caused by exposure to magic. I really enjoy Rikku's exposition on adrenalin. I like the way Rikku manages to idolize Lulu's beauty even as she's yelling at her. The lovely description of the lightning marble is another beautiful touch. You really have a knack for unique touches like these.
One more suggestion I have is to add a space between each new paragraph of your story. It makes for an easier read on an internet page.
I like the whole concept of this story though and I think you write dialogue really well.