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It was more than just "hinted" at:

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Excuse the quality; I just grabbed the pic off OM quick. They look quite dead to me... And what? If they weren't dead, he would have left them alive? I know there's always the chance of them returning somehow, and I hope they do, but I don't see how they aren't dead after that.

I think Shingo was just really old. Maybe the Exceed use had something to do with it, but I didn't get that that was the reason. She was giving him medicine to expand lifespan. When Kuro saw the age marks on his face, she was disgusted that Mikami would allow him to live so long. No one else has aged more rapidly from Exceed use in the series. It's just hard to adjust to at first. In fact, in Excel's case, she actually remains younger because of it.

I do understand the heart thing because it's typical anime corniness. They share hearts so it's touching...d'awww. But it's stupid. It's impossible to make a contract with a dead person. Hiyou even (I guess...lol) proved that with his experiments. Limbs make far more sense. The arm being able to just "fall off" was never a repeated joke. How do you figure? There was one time when Akane accidentally ripped it off, but that was fresh after the contract. Never again was it mentioned that it was fragile or at risk of coming off (and it never did again). Did I miss something?

Ah, I guess I must have modified it to what I wanted it to be and just remembered that as fact. Yeah, nevermind me. I'm going to go reread some of it so I'm not so confused anymore.