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If it was the first episode....hmm, okay, i'd send an anonymous tape to the big news corps all over the world warning of an impending shadow. That the worlds criminals would suddenly die of heart attacks. I'd say that i had a vision or something. Anyway, without anyone knowing who i am, what i sound like, and all that, people would be like "eh, whatever". Then Light would probably still use the death note, but he'd be looking for me and L at the same time. Meanwhile, i'd find a way to get a hold of Watari, and tell him who i am, and ask to merely speak with L, and explain the situation, even if it was over a webcam and i couldn't see him. I would ask him to bring Near, Mello, and Matt in to the case so all four could work together, and on top of that, my inner information would be helpful. I wouldn't say "Oh yeah Light Yagami" in the original tapes, cause who would believe me? But, once L started to trust me, and began to suspect Light, i would talk to Near, cause Near knows about death notes, and together we would confront L, and with two people with the same story on L's side, he'd come to the conclusion that Light is Kira, Misa is the second Kira, and have Mikami thrown in jail just to be safe.

Thank you, and no problem! I was happy to help. ^^

After making sure I was in the right place at the right time to witness Yagami pick up the notebook I would say "Hey! That's mine!" He would hand it over, and I would make sure to touch it so I can see Ryuk, but then say "No, wait, wrong book", then leave. Perhaps wear a hood so he doesn't remember me later.

After this I shadow Yagami along with Raye, doing my best not to let Yagami and Raye know I'm shadowing Light, and do my best to hear and see his conversations and plans with Ryuk. When Raye is about to be killed I would make sure I'm on the train and enter the carriage where Light was giving the orders (you know, the walkie talkie things) and- as it's fanfiction, put Ryuk there- ask him, "Hey, who's that?"

Light would ask who I was speaking of, and I would say "Wow, another Shinigami huh?" and when Light asked what I meant by "another Shinigami" I would say "Oh, I saw two earlier."

Calling a bluff I would convince Light there are many, many Shinigamis and Death Notes (as he doesn't yet know only 8 death Notes can be on Earth at any one time, and I doubt Ryuk would bother to inform him, nay, doubt Ryuk knew that) and then show Light a forgery of a Death Note and say, "My very own!", without allowing him to make contact with it and hide it back in my jacket. I would effectively take Misa's place from there, and when she turns up, manipulate her into thinking I'm actually Kira, using Light to get into the taskforce.

That way you could place yourself as Misa, and play Light and Misa against each other, ultimately ending however you want.

(It's terrible, I'm sorry.)